improvise/context/test-evidence/command.md

improvise-command — test evidence

Why each of these tests exists, what bug it reproduced, and what fault was injected to prove it can fail. Keyed by exact function name; a rename orphans the entry, and scripts/check-comment-runs.py --check-evidence reports that (improvise-9c8c, design-principles §7).

Entries cover tests and the helpers whose shape is the evidence.

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ui/reduce.rs

a_view_slice_command_does_not_move_the_authoritative_view_state

This test used to assert the opposite, under the name view_slice_commands_land_in_the_authoritative_view_state, and it passed against a path no host walks (improvise-s17o): apply_command's fallback arm folded a wire view command into app.view_state, the authoritative slice. Nothing in production reached it — Sessions::apply_checked folds a view command into the asking client's mirror — so the behaviour it pinned was wrong and safe only because it was dead. The model is shared, the UI is not (principle 5, gotcha 36); a client's SetStatus must not move the host's status line.

The fixture writes "host" into the status line first, so the assertion distinguishes "the fold was removed" from "the field happened to be empty" — with the default empty string, deleting the fold and leaving it in place look the same for SetStatus. Restoring reduce_view(&mut app.view_state, other) fails on the first field.

Helper — checked

Not a test — the fixture every reduce.rs test below reaches the door through.

For the tests whose subject is not staleness: a command the current state accepts, which every command built by caught_up is by construction. The tests whose subject is staleness call Checked::against themselves and assert on the Rejection, so neither kind can pass by accident through the other's convenience.

switch_view_moves_the_client_and_leaves_the_documents_default

The successor to switch_view_moves_the_document_and_the_client_together, which asserted the pre-improvise-jytm lockstep. Which view a client is on is now that client's; Workbook::active_view is the default a fresh client opens and what .improv writes. The fault it rules out is the old behaviour returning by inertia — any writer that moves the workbook's copy from a client's switch fails it.

Fault injected 2026-09-20: restoring App::switch_to_view's workbook.switch_view(name) call fails this and the_documents_default_view_outlives_this_clients_switches.

a_wire_view_switch_records_history_the_way_a_key_switch_does

A wire-driven view switch owes the same history a key-driven one leaves behind.

effect::ViewBack pops view_state.view_back_stack, and the only thing that ever pushes onto it is effect::SwitchView. While apply_command performed the switch itself — calling App::switch_to_view directly rather than running the effect — the stack stayed empty, so a client that switched over the wire could not go back, though the same two keypresses could. Wave 4 is what made that reachable: before ModelCommand there was no wire way to ask.

Nothing else in the suite could see it. switch_view_moves_the_client_and_leaves_the_documents_default checks where a switch landed; this checks what it recorded.

Bug demonstrated 2026-09-19 (Copilot, PR #11): the ViewBack left the document on "Second". Since improvise-jytm the step back is driven through effect::ViewBack rather than ModelCommand::ViewBack, which left the wire with that ticket — the stack it pops is one client's history, so a nullary wire command could not address it (gotcha 38). What the test asserts is unchanged.

switching_views_projects_the_axis_widgets_only

Switching views changes what the axis-bearing widgets read, so they are told; the list of views itself did not change, so it is not.

an_inbound_projection_is_ignored

Projections travel one way. A client that sends one back must not be able to write it into the server's state.

a_keypress_that_changes_the_model_projects_it

The key path is the model's door, and it reports the widgets it touched so a host can project them.

a_view_only_keypress_projects_nothing

A keypress that moves only this client's cursor reaches no other client (gotcha 3).

every_model_command_names_an_effect_that_declares_a_model_change

The derivation, stated as a check. This is what "derived, not invented" means operationally (doctrine(derive_the_enum_from_the_effects)). A variant pointed at a ViewOnly effect would be a wire message that changed the document for the client that sent it and for nobody else — Dirty::of would report NOTHING, and no projection would ever correct the others (gotcha 35).

Helper — model_app

Not a test — the fixture the three every_model_command walks run against.

app() with a cell actually stored, so a surrogate for it resolves through the index rather than only through the layout. The walks over every_model_command run against this: a SetCell whose record cannot be resolved produces no effects, and every one of those tests asserts on the effects, so the fixture has to name something real or they would all fail for a reason that is not theirs.

no_model_command_commits_another_clients_session_local_state

improvise-3x3u: the wire vocabulary is the addressable model effects, not all of them.

Sessions::apply routes a Command::Model with no SessionId, which is right exactly when the change is addressed to no one session (gotcha 36). ApplyAndClearDrill and AcceptWizardModel are addressed to precisely one, and it is not the sender: an unaddressed ApplyAndClearDrill commits another client's private typing (improvise-tom1) and an unaddressed AcceptWizardModel swaps the document out of a dialogue the sender never opened. Both were demonstrated against Sessions::apply before the variants were removed.

Re-adding either variant means making it addressed — carrying a SessionId, with the staging map moved onto the session — not restoring the nullary form this test forbids.

SESSION_LOCAL_EFFECTS is the list of the two effect names.

the_session_local_effects_are_real_model_effects

The guard above is only worth having while the effects it names still exist and are still model changes — otherwise it would pass by naming nothing (doctrine(check_the_checker)).

the_effect_name_reader_reads_all_three_debug_shapes

The reader above is itself a check, so it gets the injection too (doctrine(check_the_checker)): one effect of each Debug shape, because the leading identifier is followed by a different delimiter in each — nothing, (, and {.

Helper — effect_type_name

Not a test — the reader that test injects against, used by the two guards above.

The type name of an effect, read off the leading identifier of its derived Debug. Effect is a trait object, so std::any::type_name is not reachable through it; every effect derives Debug, and a derived Debug opens with the type's own name for a unit struct, a tuple struct and a braced struct alike. So the name is already on the value — this only reads it back.

every_model_command_names_the_effect_of_the_same_name

The mapping is name-for-name by construction, so state that: every ModelCommand variant names the effect that carries its own name.

This is the check that separates an effect from its inverse. every_model_command_names_an_effect_that_declares_a_model_change cannot: HideItem and ShowItem, AddItem and RemoveItem, CreateView and DeleteView, SetCell and ClearCell are each two effects of the same EffectKind, so a swapped pair clears that bar while inverting what the wire message does. And a_model_command_lands_where_its_keypress_lands cannot reach them either — most variants have no single keypress to compare against, so it drives three of twenty-eight.

Proven by injection 2026-09-19: HideItem → effect::ShowItem, CreateView → effect::DeleteView and AddItem → effect::RemoveItem all left cargo test --workspace at 918 passing, 0 failing. This test fails on each.

The name-for-name rule is a design constraint, not a coincidence, and it is what makes the derivation checkable at all (doctrine(derive_the_enum_from_the_effects)): a variant whose effect must be named something else is a variant that stopped being derived, and it should fail here and be argued about rather than pass quietly.

the_enumeration_names_each_variant_once

The every_model_command list covers the enum once. A duplicated entry would leave a variant untested while the count still looked right.

Helper — every_model_command

Not a test — the enumeration that test, and the three walks above it, cover.

One value per ModelCommand variant. The enum is the derived set, so a test that walks "the set" has to walk all of it. variant_name is the completeness guard: it matches exhaustively, so a variant added to the protocol fails the build here until it is listed in this vector as well.

a_model_command_lands_where_its_keypress_lands

The point of the whole ticket: a model Command applied authoritatively leaves the document exactly where the keypress bound to the same mutation leaves it. That equivalence is what lets the two entry points share one door.

The Dirty is compared too, because a host with connected clients projects off it: a command that produced the right document and the wrong Dirty would leave every other client stale.

Its pivoted() fixture has two categories on different axes, because the one-category fixture cannot tell a transpose from a cycle: with only Region on Row, swapping the axes and rotating Row→Column land on the same view, and an equivalence test that cannot distinguish the wrong effect from the right one proves nothing (doctrine(check_the_checker)).

a_stale_wire_model_command_cannot_be_applied

A wire Command::Model cannot reach the document without its basis being checked (ADR 001, improvise-ondj).

Bug demonstrated 2026-09-19: reduce_full and apply_command — the functions this module's own doc points every future host at, the browser worker included — took a &Command and matched Command::Model { cmd, .. }, so a batch composed against a document that had since moved landed anyway. The .. discarded the basis with no compile error.

The regression guard is the signature, not this test. Both now take a Checked, whose only constructor runs stale_basis, so the version of this test that reproduced the bug — reduce_full(&mut app, &wire) with a stale wire — no longer compiles, and reduce_full itself no longer exists (improvise-s17o: having no caller was the condition that let the defect hide in the first place). What is left to assert is that the constructor refuses, and that refusing leaves the document alone: there is no half-applied batch, because the type is obtained before anything is applied.

a_model_commands_dirty_comes_from_its_effects_kind

The Dirty a model command reports is the one its effect declares, not a second table kept alongside the enum (gotcha 35).

a_wire_model_command_marks_the_document_dirty

improvise-gv9r. The bug: a client's edit left the authoritative document reporting clean, on every transport.

MarkDirty used to be a separate effect that the command layer composed alongside the mutation, and improvise-kmgs derived ModelCommand from the effects one at a time — so the wire got SetCell without its companion. A derivation correct per element and wrong for a set. "Save if dirty" was therefore a trigger that never fired, and a host built on it loses every client edit silently; the browser worker found that out the hard way and had to over-approximate off the command variant instead.

The fix is the unrepresentable one: dirtiness is raised by App::apply_effects off the same EffectKind the Dirty and the two basis tiers are read from (gotcha 40), so no path can compose a document change without it. There is nothing left to forget, and MarkDirty is gone rather than left as a second answer.

The three levels are asserted separately because they are three different effects: an effect that is only ever reached by one of them would otherwise pass on a sibling's evidence. The negative arm — that a ViewOnly effect leaves the document clean — is a_view_only_batch_leaves_the_document_clean in ui/app.rs, and it has to live there: a view-slice Command never reaches apply_effects at all (apply_command folds it straight into ViewState), so a negative written against this door would pass however unconditional the raise was.

a_wire_view_switch_leaves_the_document_clean

The inversion of a_wire_view_switch_marks_the_document_dirty, which asserted the pre-improvise-jytm behaviour. A switch used to move Workbook::active_view, which persists as the Initial View line, so it really did put the document out of step with the file. It no longer moves any document state, so there is nothing to persist and the browser worker's persist trigger (improvise-gv9r) must not fire.

The direction matters: the worker asks the document's dirty flag rather than a table of variants, so a switch that dirtied spuriously would cost an OPFS rewrite of the whole document per keystroke (gotcha 41). This is the negative arm — the positive one, that a real model command does dirty, is the loop above it.

The consequence to keep in view is a product one, filed as improvise-azhb rather than decided here: a TUI user who switches view and saves now writes the Initial View the document was loaded with.

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ui/effect.rs

the_document_surface_runs_on_a_non_filesystem_backend

improvise-fuj: every effect that touches a document goes through App::storage, so the whole save/load/import/export surface works on a backend that is not a filesystem. This is what a browser worker supplies (improvise-i34); here MemoryFs stands in for it, and the test would fail if any of these effects reached for std::fs.

view_only_effects_leave_shared_state_alone

improvise-mae: a declared kind() is only worth having if it matches what the effect actually does. Rather than restate each declaration, this applies the effect and checks the shared state it claims not to touch is genuinely unchanged — a ViewOnly effect that quietly wrote a cell would never reach connected clients.

No exemptions: since improvise-8uz moved the cursor out of View, SetSelected/SetRowOffset/SetColOffset touch nothing shared, so every effect in the list must clear the same bar.

structural_effects_declare_their_kind

The structural effects, each pinned to the kind it must declare.

The bug this guards: AddCategory and RemoveCategory declared CellData, but both go through Workbook, whose on_category_added / on_category_removed rewrite every view's axis map. That is a re-layout, so a client told only "cell data changed" would keep rendering the old grid shape. The two "actually change" tests beside it cannot see this — a category change moves model data and view config, so it passes either list. Only pinning the declaration does.

add_category_past_the_limit_reports_the_failure

Bug (improvise-djn): AddCategory discarded the Result from Workbook::add_category, so pressing n in the category panel with the 12-regular-category limit already reached reported the optimistic Added category "X" its command had queued and added nothing. The effect runs last precisely so its failure can overwrite that message (improvise-6t3) — it just never wrote one.

add_formula_adds_target_item_to_category

Regression: AddFormula must add the target item to the target category so the grid layout includes cells for it. Without this, a formula like Margin = Profit / Revenue would be registered but invisible in the grid.

abort_chain_short_circuits_apply_effects

AbortChain must cause subsequent effects in the same apply_effects batch to be skipped, and the flag must reset so the next dispatch starts clean.

save_without_file_path_aborts_the_chain

BUG (improvise-8tk): Save reported failure only through the status bar, so the Quit that wq/ZZ chains after it landed anyway and the whole session was discarded without the user seeing the error. A failed save must abort the rest of the batch.

failed_save_as_aborts_the_chain

BUG (improvise-lae, found reviewing it): improvise-8tk fixed one arm of a rule with two. Save aborts the batch when the write fails; SaveAs reported the error to the status bar and let the batch continue, which is the same silent discard 8tk closed — the shape doctrine(one_arm_of_a_symmetric_rule_is_evidence_about_that_arm_only) names, and the three tests above it are evidence about Save only.

It is reachable today: the : line composes with ., so :w /bad/path . q builds exactly this batch, and ExecuteCommand concatenates the two commands' effects into one. improvise-lae's save prompt also commits to SaveAs, so the arm was about to carry more traffic.

Note what the abort does and does not buy the prompt: the typed path survives for the user to correct, but the message does not reach the screen, because draw_bottom_bar draws the minibuffer instead of the status line while a prompt is open. That is pre-existing and reported separately; the data-loss half is what this fixes.

successful_save_as_lets_the_chain_continue

The positive arm: a SaveAs that succeeds must not abort, or every :w <path> would swallow whatever followed it. Without this, "always abort" passes failed_save_as_aborts_the_chain.

enter_edit_at_cursor_uses_target_mode_not_app_mode

EnterEditAtCursor must use its target_mode field, not whatever app.view_state.mode happens to be when applied. The previous implementation branched on app.view_state.mode.is_records(); the parameterized version trusts the caller (keymap or composing command).

enter_edit_at_cursor_reads_post_batch_layout

improvise-ehz: the edit buffer must be loaded from layout state that already reflects earlier effects in the same batch. apply_effects only rebuilds after the batch, so a preceding RebuildLayout is what makes the cursor resolve to the post-mutation cell. Removing the row-0 item slides a different cell under the cursor; the buffer must show that cell's value, not the pre-batch one.

set_buffer_does_not_special_case_the_search_name

SetBuffer no longer treats "search" as an alias for the search query (improvise-gdl): it is a plain buffer write whatever the name says, and the query is reached through SetSearchQuery. Commands pick between the two in cmd::core::write_buffer.

apply_and_clear_drill_coord_rename_moves_every_measure_in_the_row

improvise-c5v: in wide format a records row groups one cell per measure, so renaming a coordinate has to move every one of them. The long-format code moved only the cell under the cursor, which would now split the row in half.

import_headless_csv_short_rows_surface_warning_in_status

BUG (improvise-k8i): ImportJsonHeadless parsed CSVs with parse_csv, which sends short-row warnings to stderr — invisible inside the TUI. Short rows must surface in the status message, naming the affected column, never silently succeed.

Note — the drill tests drive apply_effects, not apply

Several ApplyAndClearDrill tests call app.apply_effects(vec![...]) rather than effect.apply(&mut app). That is deliberate: the dirty flag is raised off the EffectKind now (improvise-gv9r, gotcha 45), so driving the effect directly would assert a flag nothing in production sets.

every_file_writing_effect_aborts_the_batch_on_failure

improvise-z05r, the third instance of one rule: improvise-8tk fixed Save and not SaveAs; PR #27 found SaveAs; ExportCsv was the arm still missing, so a Binding::Sequence or a : line composing an export with anything after it continued past a failed write.

The fix is structural rather than a third patch: all three now report through one report_file_write(app, outcome), which writes the status message either way and sets abort_effects on Err. That is not sufficient by itself — a fourth writer could still be written without calling it — so the test names the three, and a fourth has to come here and be named. Naming is the mechanism; the shared function is what makes the naming cheap to honour.

Each case drives App::apply_effects with [writer, ChangeMode(Quit)] against a path under a directory that does not exist, and asserts both last_batch_aborted() and that the trailing mode change never landed.

Injected fault: drop app.abort_effects = true from report_file_write. Fails by name, along with the four older abort tests — which is the point: they and this one now share one mechanism.

Injected fault, narrower: route ExportCsv around the reporter and write its status directly. Only this test fails, naming ExportCsv, with 460 passing — the exact state the ticket describes.

a_batch_that_writes_nothing_leaves_the_document_clean

improvise-748f. ViewNavigate { forward: false } emits ApplyAndClearDrill unconditionally, and that effect returns immediately when drill_state is None. It declares Model(CellData) regardless — Effect::kind takes no App, so the classification cannot be state-dependent — so apply_effects raised model_state.dirty on a document nothing had touched. Press view-back, press :q, and the quit is refused; autosave arms thirty seconds later.

The older test here, start_drill_and_apply_clear_drill_with_no_edits, pinned the wrong behaviour in an assertion message arguing it was the same over-approximation a failed LoadModel gets. It is not: a succeeding LoadModel leaves a document that really does differ from the file its path names, and that remains declared. This was a wrong answer to no divergence at all.

a_cell_data_effect_no_raw_cell_records_still_dirties_the_document

The guard against the cure improvise-748f proposed, which is unsound. The ticket suggested raising dirty off bump_changed_records' per-record fingerprint of the raw cells, on the reading that CellData means cells. Five of the ten CellData effects change the document without writing a raw cell: AddItem, AddItemInGroup and SortData (which reorders — and .improv carries the order), and AddFormula and RemoveFormula, whose results live in Model::formula_cache and not in the store. Observing raw cells would have left every one of them reading clean, which is a lost edit — the opposite direction from the spurious prompt it was fixing.

So the observation is document_content — document_config plus the cells in iteration order and the formulas — and only the CellData arm reads it. The LayoutGeometry arm keeps the narrow document_config. A first version pointed both arms at the wide fingerprint, and review benchmarked what that costs at 18,000 cells in release over 200 batches after warm-up: one SetCell batch 8.07 ms → 10.21 ms (+27%, a constant factor on a path that was already O(cells) through record_fingerprints), but one SetNumberFormat batch 78 µs → 2.63 ms, 33× — an asymptotic change from O(config) to O(cells + config) on the held-key path that [/], axis cycle, hide/show, group toggle, transpose and prune toggle all take. ~380/sec where they had been ~12,800/sec. The split costs no correctness because all five blind effects are CellData; that every LayoutGeometry effect really is visible to the narrow fingerprint is asserted separately, and not left as a reading.

WholeWorkbook stays declared, and that is the remaining over-report: a LoadModel whose incoming content happens to be identical still marks the document dirty, and a failed one does too. Both cost one redundant write.

The SortData case needs two cells inserted out of sorted order, or it reorders nothing and its case asserts the opposite of what it names. The AddFormula case removes the fixture's formula first, or adding it is a no-op.

Injected fault: narrow document_content back to the configuration it hashed before. Fails naming SortData, with five other dirty-flag tests.

every_layout_geometry_effect_is_visible_to_the_narrow_fingerprint

The guard that makes the two-fingerprint split safe rather than a trap. Splitting CellData (wide) from LayoutGeometry (narrow) is only sound while every effect wearing LayoutGeometry writes something document_config hashes; an effect it cannot see would leave the document reading clean forever, which is the same lost-edit failure the wide fingerprint exists to prevent, arriving by the other door.

The thirteen are enumerated by brace-matching the impl Effect blocks, not by grepping — the naive grep reports fourteen, picking up HelpPageSet, which is ViewOnly and is simply the last pub struct before #[cfg(test)] (gotcha 38 names this exact artefact). Each case asserts its effect really declares Model(LayoutGeometry) before asserting anything else, so a reclassified effect fails here rather than silently leaving the list unrepresentative.

Four cases carry a prepare step run directly against the workbook and not through an effect, because the interesting state is the precondition. What makes such a case honest is the no-op-fails property: without its precondition the effect writes nothing, an unwritten document leaves the fingerprint unmoved, and the case therefore fails rather than passing vacuously.

Measured, by deleting each prepare in turn, two of the four are load-bearing in exactly that way and two are not:

• DeleteView needs a second view; without it, dirty reads false and the case fails.
• ShowItem needs a hidden item; without it, dirty reads false and the case fails.
• ToggleGroup is not load-bearing here. ToggleGroup::apply calls toggle_group_collapse unconditionally — its membership check gates only reset_cursor — so collapsed_groups moves whether or not the group is real, and the case passes with the prepare deleted. The step documents the realistic input; it is the cursor arm, tested elsewhere, that needs a real group.
• TransposeAxes is not load-bearing either, and its step is literally a no-op: View::on_category_added already assigns the first regular category Row and the second Column, so the fixture arrives with Type/Month differing. The step states the precondition rather than establishing it, and the case passes without it.

dirty is cleared after prepare and before the effect. That clear is defensive hygiene for a future prepare that goes through apply_effects, and it is not what makes any case honest today: dirty is raised only inside App::apply_effects, and no prepare in this test runs an effect, so no preparation can set it. Demonstrated by removing the clear and blinding document_config to 0 together: all thirteen cases still read dirty == false. An earlier version of this entry claimed the opposite — that without the clear "all four cases pass against any fingerprint" — and measurement contradicts it.

Injected fault: document_config returns 0. Fails, with five other dirty-flag tests across three files. Injected fault, narrower and the one that proves the test is pointed at the right thing: drop view.number_format from document_config. Fails naming SetNumberFormat, with only one other test failing anywhere.

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ui/app.rs

model_state_and_view_state_are_constructible

improvise-3vr: ModelState and ViewState are the named slices of App state introduced by the vb4 refactor. Step 1 only requires that the types exist and are constructible; subsequent steps move fields in.

Note — the ViewState: Send static assertion

improvise-h0b: ViewState is the portable slice — the one thing serialized into the Snapshot a newly-connected client receives (improvise-q08). Send is the cheapest compile-time proxy for "portable": a field that cannot cross a thread boundary certainly cannot cross a serialization one. Before h0b this failed to compile, because drill_state: Option<DrillState> holds an Rc<Vec<RecordRow>>. Session-local state lives on App instead. It is a const _: fn() in the test module, not a named test, so it has no heading of its own.

app_view_state_owns_ui_session_fields

improvise-ew0: ViewState owns the UI session slice — mode, status, search, panels, navigation, drill, yanked, buffers. App accesses them through view_state.

view_state_survives_a_json_round_trip

improvise-a2x: ViewState is what a newly-connected client receives as a Snapshot (improvise-q08), so it has to survive a round trip through the wire format. Populated with non-default values in every field family — a mode carrying a MinibufferConfig, a navigation frame, a yanked cell, a keyed buffer, an armed prefix — because a roundtrip of ViewState::default() would pass while proving nothing.

the_documents_default_view_outlives_this_clients_switches

The replacement for the_two_active_view_names_never_diverge, and it is a replacement rather than a deletion on purpose: improvise-jytm makes the two copies diverge deliberately, and a deliberate split with no guard is how one silently regresses into the other.

What it pins, all four of which have a silent failure mode. A switch moves the client's copy and not the document's default. App::view() — what rebuild_layout and cmd_context read — follows the client, so the split is observable in the layout and not merely in a string. A switch to a view the document does not have moves nothing, because the slice keys ViewState::cursors and grid_in indexes the workbook by it. And install_workbook still seats this client on the incoming document's default and clears the cursor map, since the outgoing document's view names mean nothing in the new one (gotcha 24).

Fault injected 2026-09-20: App::switch_to_view calling workbook.switch_view(name) again fails the second assertion.

moving_the_cursor_does_not_touch_the_shared_workbook

improvise-8uz: moving the cursor must leave the shared document untouched, so that one client scrolling cannot move another's viewport. Before the move these three effects wrote into model_state.workbook, and this assertion would have failed.

app_owns_the_session_local_fields_not_view_state

improvise-h0b: the two session-local fields moved off ViewState and onto App. Each fails both membership tests: ImportWizard is an improvise-io type (not portable) and a half-open dialog (not snapshot-safe); DrillState holds an Rc (not portable — see the Send assertion) and stages uncommitted edits (not snapshot-safe).

staged_drill_edits_are_unreachable_from_view_state

improvise-h0b: a fresh client must not inherit another session's staged, uncommitted drill edits. Those edits are unreachable from ViewState, so a Snapshot built from it cannot carry them.

switching_views_restores_each_views_own_cursor

improvise-8uz: the cursor is per view. Written as a characterization test against the old View-owned fields and kept verbatim across the move to ViewState::cursors, because a refactor about where state lives must not change what the app does.

the_improv_format_does_not_persist_the_cursor

improvise-8uz: the ticket asserted these fields were "PERSISTED in .improv". They were not — the grammar has no cursor production and the writer emitted none. That is what made the move free: there was no restore-where-you-left-off behaviour to lose, and no format change to make. The assertion still guards the format going forward.

entering_records_mode_sorts_existing_data

Pressing R to enter records mode sorts existing data by CellKey, so display order is deterministic regardless of insertion order.

Note: this test's doc comment carried a stray first line from a neighbouring test ("Regression: pressing o in an empty records view should create the") that was never finished. It was dropped in the comment-cap wave; the sentence above is what the test actually asserts.

the_new_category_column_creates_a_category_and_its_column

improvise-2zq step 2: the +cat column creates a category and its column, without leaving the grid.

Driven through real keys. The assertion that earns its place is the last one: the new column is usable immediately, which is the whole point — records_columns filters on kind.is_regular() and never requires items, so a category with no items is a column the moment it exists.

enter_on_plus_cat_in_the_last_row_does_not_reopen_category_creation

improvise-2zq, found by Copilot on PR #27: creating a category from +cat shifted the grid under the advance that followed it.

add_category inserts the new coordinate column immediately before +cat, so every column index from +cat rightwards moves by one. CommitAndAdvance then computed its advance from ctx, whose layout is the pre-commit one (gotcha 23: apply_effects rebuilds only after the whole batch). On the last row Enter wraps to the next column — and the index it wrapped to was +cat again after the rebuild.

So the failure is not "lands one column off". It reopens category creation: a user naming two categories in a row never reaches the column they just made, and gets an unbounded +cat loop instead. The last row is the arm that wraps, so it is the arm tested.

tab_on_plus_cat_does_not_reopen_category_creation

The same stale index reached Tab, which advances right rather than wrapping. Both directions read the pre-commit layout, so both are covered — one arm of a two-arm rule is the shape doctrine(one_arm_of_a_symmetric_rule_is_evidence_about_that_arm_only) names.

the_new_category_column_creates_the_column_but_writes_no_coordinate

Committing the +cat name creates the column and does not write the coordinate on the row the cursor was on.

The deliberate answer to the ticket's third question, and the reason is gotcha 1: setting a coordinate moves every measure cell in the row. That is a data migration across N cells, and attaching it to the same Enter that made a schema change — in an app with no undo — is a bigger single keystroke than the measure + asks for. The follow-up edit is one column away.

the_new_category_column_refuses_past_the_limit_where_the_user_can_see_it

MAX_CATEGORIES is a reachable refusal on this path, and the refusal has to be visible.

draw_bottom_bar renders the minibuffer instead of the status line, so a message written while an editor is open is never drawn. The commit path therefore drops out of editing before running the add, and AddCategory aborts the batch on refusal so the trailing EnterEditAtCursor cannot re-open the editor over the message.

The limit itself is checked in exactly one place — Model::add_category — and nothing here restates it; the test fills the document to the limit by asking for categories until it refuses.

naming_a_category_in_records_mode_does_not_eject_the_user

improvise-2zq step 1: naming a category in records mode used to eject the user from the grid they were standing in.

Records mode is _Index on Row and _Dim on Column, with every regular category on Axis::None. View::on_category_added seated a new regular category on the first axis no regular category held — Row — and bumped the virtual out of the way to get there. So N, a name, Enter moved _Index off Row, records detection stopped matching, and the grid silently became a pivot.

Driven through the real keys rather than through on_category_added, because that is how the bug was found and it is what makes this evidence rather than a restatement of the fix.

i_and_o_diverge_on_an_empty_records_grid

improvise-6sq: i and o want opposite behaviour on an empty records grid, which is why edit-or-drill and enter-edit-at-cursor cannot be merged into one command. Entering records mode puts every regular category on Axis::None, so EditOrDrill's aggregation check is true unless the cursor is on a synthetic record — and on an empty grid there is none, so i reports "cannot drill". o must still open an edit there (see add_record_row_in_empty_records_view_creates_first_row), so routing it through the same drill check would break it.

improvise-ct5 changed the message and deliberately not the divergence. The outcome — i opens no editor where there is nothing to edit — is right, and o one key away is the affordance for adding a row. What was wrong is that the message said "cannot drill", a pivot-mode concept in a mode whose model is "a table I type into". Making i behave like o here (the ticket's option (b)) would collapse exactly the divergence improvise-6sq exists to record, so the assertion still pins that i does not enter editing; only the wording assertion moved, and it now also names what must not appear.

the_empty_grid_message_is_not_the_records_one_in_pivot_mode

The negative arm of improvise-ct5, and the reason the wording is keyed off the mode the keymap supplies rather than reworded once.

Without this, "say the records sentence everywhere" passes i_and_o_diverge_on_an_empty_records_grid — and it would promise o, which adds no row in pivot mode, trading one misleading message for another.

The fixture is an empty pivot grid that still counts as aggregated: Region on Row with no items makes cell_key() None, and Extra cycled to Axis::None is the regular category that makes EditOrDrill take the drill branch.

blank_model_bootstraps_its_first_measure_from_the_new_measure_column

A blank model has no categories and no measures, so wide format shows exactly the two creation columns. Typing a name into the new-measure one creates the measure, which gives it a real column to type values into — records mode stays self-sufficient for onboarding (improvise-c5v).

improvise-2zq widened that: a blank grid now offers +cat beside +, so both halves of a schema can be bootstrapped without leaving the grid. This test still pins the measure half; the category half is the_new_category_column_creates_a_category_and_its_column.

Inside it, the cursor lands at column 0, which in a blank grid is now +cat rather than + (improvise-2zq) — a record needs coordinates before it needs measures, so that order is deliberate. o leaves the test editing column 0, so the Esc before moving to the new-measure column is load-bearing.

ctrl_w_prompts_for_a_path_and_a_failed_write_reports_it_where_it_shows

improvise-lae end to end: Ctrl-W, a path, Enter.

The failing case is the interesting one, and it pins an ordering (found by Copilot on PR #27). SaveAs aborts the batch on a failed write, so whatever CommitSave emits after it is skipped. A first version emitted [SaveAs, ChangeMode(Normal)], which left the app in SavePrompt — and draw_bottom_bar renders the minibuffer instead of the status line, so the "Save error:" the effect had just set was never drawn. The save appeared to do nothing whatsoever.

Leading with the mode change puts the error in Normal, where the bottom row is the status bar. This test therefore asserts the mode left the prompt, which is the opposite of what it asserted before — deliberately, and it is the only thing keeping the two effects in that order.

every_minibuffer_mode_entered_from_records_returns_to_records_normal

Copilot round 2 reported that Ctrl-W from a records grid returns to Normal, losing the records keymap while the layout is still a records layout. It is right, and it is a class: every minibuffer mode reachable from RecordsNormal exited to Normal, because the exit was a constant baked into TextEntry::new(..., "normal") and into each commit command.

The class predates SavePrompt: : and Ctrl-E do exactly the same thing and always have. That is why the fix was not "make SavePrompt remember its origin" — that would leave two of the three arms broken, which is the shape doctrine(one_arm_of_a_symmetric_rule_is_evidence_about_that_arm_only) names.

The binding table is not involved and was checked rather than reasoned about: an IMPROVISE_BINDING_DUMP diff shows RecordsNormal gains Ctrl-W and loses nothing, so with_parent inheritance (gotcha 13) is intact. What was lost is the runtime mode.

improvise-sihr fixes it by observation rather than memory, which is what took the exit out of the keymap entirely: App::apply_effects calls settle_normal_mode after rebuild_layout, so the mode a batch ends in matches the grid it ends on and nothing carries an entry mode. The ticket proposed the opposite — carrying the entry mode on MinibufferConfig — and that was declined for a concrete reason beyond doctrine: carried generically it would also return CategoryAdd to CategoryPanel, a behaviour change nobody asked for, while carried for only three modes it is the one-arm shape again.

This test still drives both exits — Esc and Enter — where the original drove only Esc, because they reach the mode through different effects (ChangeMode and ReturnToNormalUnlessMoved) even though one settle now covers both.

The first version of the fix resolved per-effect and was wrong in a new direction, which the adversarial review of PR #91 caught. Set at the moment the mode is written, the resolution reads the batch's opening layout (gotcha 23) — so a batch that crosses settled against the grid it started on. See a_batch_that_leaves_records_and_then_sets_normal_lands_in_normal.

Injected fault: settle_normal_mode()'s call site removed. Exactly three tests fail across the whole workspace — this one, the :-line sibling and the crossing test — which is the per-edit coverage question answered by there being one edit.

a_colon_line_run_from_records_settles_back_into_records_normal

The sibling above cannot see ReturnToNormalUnlessMoved, and that was found by injection rather than by reading: forcing only that effect back to AppMode::Normal left the sibling green. An empty : line — which is what pressing Enter straight after : gives — takes ExecuteCommand's raw.is_empty() branch and settles through ChangeMode, so the effect that settles every line that actually runs was untested.

So this test types a line. page-next rather than yank, because the script parser requires coordinates for yank and an unparseable line takes the error branch — which settles through ChangeMode too, and would have made this test a second copy of its sibling while looking like a new case. The setup assertion that parse_line_with accepts the line is there for exactly that: a future rename of the command would otherwise turn this test vacuous silently.

Injected fault (against the per-effect version of the fix): ReturnToNormalUnlessMoved forced to AppMode::Normal. This test failed, left: Normal right: RecordsNormal, while the sibling stayed green — which is the point, and is how this test came to exist. Against the shipped end-of-batch settle the discriminating injection is removing settle_normal_mode()'s call site, which fails this test with the same values.

a_batch_that_leaves_records_and_then_sets_normal_lands_in_normal

New wrong state introduced by PR #91's first attempt, found by the adversarial review and verified by reverting only ui/app.rs and ui/effect.rs to base and re-running the identical probe:

:toggle-records-mode . enter-mode normal	base	#91 round 1
from a pivot grid	Normal (the sihr bug)	Normal still
from a records grid	Normal (correct)	RecordsNormal

The cause is gotcha 23: resolving at the moment the mode is written reads the batch's opening layout, so a batch that crosses settles against the grid it started on. :toggle-records-mode is the case the round-1 prose offered as proof that the staleness was a property — and it is not, because ToggleRecordsMode emits change_mode(RecordsNormal) explicitly on entry and ViewBack restores the pushed mode on exit, so the resolution's answer is overwritten in both directions and the case exercises nothing. That is a careful comment standing in for a test, in the PR that recorded doctrine(an_injection_that_does_not_fail_names_an_untested_path).

The fix moves the resolution to one place, settle_normal_mode after rebuild_layout, where there is no crossing case left to get wrong.

This test drives both directions deliberately. One direction alone is satisfied by a settle that only ever answers one way, and the pivot→records direction was also wrong in round 1 for the mirror-image reason.

a_wizard_abandoned_over_a_records_grid_returns_to_records_normal

CloseWizard writes view_state.mode directly rather than through an effect that could resolve it, so under round 1's per-effect design it was a third writer needing its own edit — and reverting that edit alone left the whole workspace green, which the review filed. The end-of-batch settle removes the per-edit question entirely (there is one edit), and this test covers the path the review named: the wizard abandoned over a records grid.

It drives CloseWizard as an effect rather than typing :import, because the wizard needs a real CSV and the contract under test is one batch's settling, not the import pipeline.

a_failed_colon_write_lands_in_normal_where_its_error_is_drawn

A failing : command must not strand the user in CommandMode.

Found by Copilot on PR #27, and it is the sharper form of gotcha 21. The abort is supposed to stop the chain — :w /bad . q must not quit. What it must not also stop is the bookkeeping: the mode restoration that ExecuteCommand appends. Aborting before that ran left the app in CommandMode with the stale command line still drawn, and draw_bottom_bar renders the minibuffer instead of the status line — so the "Save error:" set one effect earlier was invisible. A failed save looked exactly like a key that did nothing.

The answer is ordering, not classification: a batch puts its tidy-up where an abort cannot reach it, which is in front. No effect needs a new "runs anyway" flag, and the rule generalises to any command composing bookkeeping with something that can fail.

Both arms are here because they fail in different places: no path at all is Save's guard (aborting since improvise-8tk, so this arm was broken before that branch), and a bad path is SaveAs's write (aborting since f0f400d).

a_failed_colon_write_still_refuses_to_quit_after_it

The other half of the same rule: the abort must still stop the chain. :w <bad> . q is improvise-8tk's data loss spelled on one : line, and moving the mode restoration to the front must not have bought the fix above at the price of that one.

a_sequence_whose_first_step_aborts_does_not_run_the_second

Gotcha 21 claims a failed Save aborts across Binding::Sequence steps as well as within one command's effect list. Nothing tested it: no shipped Sequence has a first step that can abort, and ZZ is a prefix resolving to the single wq command, so prefix_z_then_z_reaches_save_and_quit exercises the within-a-command case only. The claim was structurally true and unguarded.

It is true because Keymap::dispatch flattens a Sequence into ONE effect vec and App::apply_effects runs that as one batch, breaking on abort_effects. It is load-bearing, because it is the reason a [w, force-quit] Sequence would be safe — the argument the grooming note unblocked on.

And it is exactly what improvise-0hb would remove. Command-level transactionality means applying effects between steps, and apply_effects resets abort_effects at entry and again on abort, so step 2 would be dispatched into a fresh batch with the flag cleared — the Quit after a failed save would land, silently reopening the improvise-8tk P1 data loss. This is the guard that would catch it.

unmatched_key_after_prefix_is_swallowed

A prefix is consumed whether or not the next key matches, so a stray key after g does nothing rather than being treated as a fresh normal-mode binding.

renaming_a_coordinate_moves_every_measure_in_the_row

improvise-c5v: a records row groups one cell per measure, so editing a category column has to move every one of them. The long-format code moved only the cell under the cursor, which in wide format would tear the row apart — half its measures under the old coordinate, half under the new one.

leaving_records_mode_cleans_empty_key_cells

improvise-3zq (bug #2): AddRecordRow creates a cell with an empty CellKey when no Page-axis categories supply coords — that cell serialises as = 0 in .improv and re-appears on every records toggle. Leaving records mode must clean up any such meaningless records (the inverse of the SortData that runs on entry).

enter_at_bottom_right_of_records_view_exits_editing

improvise-3zq (bug #1): Enter on the bottom-right cell of records view should commit and leave edit mode. Previously CommitAndAdvance pushed an EnterEditAtCursor effect unconditionally, so the cursor stayed put and editing re-opened on the same cell.

Helper — records_model_with_two_rows

Not a test — the fixture for every Tab/Enter boundary test in this file.

A records-mode app with two data rows, for testing Tab/Enter behaviour at boundaries. In wide format a row is one coordinate tuple, so the two rows are two Region items, each carrying both measures (improvise-c5v).

tab_on_bottom_right_of_records_inserts_below

improvise-hmu: Tab on the bottom-right cell of records view inserts a new record below and moves to the first cell of the new row in edit mode.

Note — the wizard_key_tests module

Behavioural coverage for the import wizard's key handling (improvise-2wm). The wizard is driven entirely through handle_key, so these tests pin the user-visible contract independently of whether the step/key decisions live in an effect or in a command.

name_model_typing_edits_the_name_and_enter_confirms_the_import

Enter at NameModel builds and parks on the Done confirmation; a second Enter there installs the workbook and closes the wizard (improvise-907, from improvise-c6j).

review_proposals_blocks_when_no_category_is_accepted

BUG (improvise-907, from improvise-gfq): an import whose fields all classify as Measure used to walk the user through DefineFormulas and NameModel before build_model refused. The refusal now happens at ReviewProposals, where c can still change a field's kind.

left_walks_back_through_the_steps_taken

BUG (improvise-907, from improvise-o1c): reaching a later step and wanting to change a field kind meant Esc, which discarded every decision. Left walks back through the path actually taken (gotcha 22).

done_esc_returns_to_name_model_and_drops_the_built_model

Esc at Done goes back to NameModel rather than discarding the import, and the workbook built for confirmation is dropped so a later accept cannot install stale field decisions.

Note — the basis_tests module fixture

The two lower tiers of a batch's basis (ADR 001, improvise-xwi6). Fixture note: two regular categories, two measures and two records, deliberately. A one-category fixture cannot tell TransposeAxes from CycleAxis, a one-record fixture cannot tell "the record it touched" from "every record", and a one-measure fixture cannot tell a record from a cell — the three distinctions these tests exist to draw.

a_records_two_measures_share_one_record_key

The record is the coordinate tuple with _Measure gone, and both measures of one coordinate name the same record, which is the whole point of the unit.

a_layout_change_moves_the_layout_version_and_no_record

Tier 2 moves, tier 3 does not. A shape change invalidates every in-flight CellData batch through layout_version alone; it must not also masquerade as a write to any record.

Fixture repaired under improvise-75ld: it asked for SetAxis { category: "Month", axis: Axis::Column } against a fixture where on_category_added had already seated Month on Column — so the "layout change" was a no-op, and the test passed only because the tier was bumped off the declaration. It now asserts the axis before and after, and asks for Axis::Row. This is the failure that turned "reclassify three effects" into "stop reading the tier off the classification". Injected fault: the same as both_shape_counters_move_for_every_reshaping_command — fails naming SetAxis.

a_loaded_document_is_dirty_because_its_path_still_names_the_old_one

A loaded document is dirty, and that is a behaviour change with a reason (improvise-gv9r, raised by the independent review of PR #26).

LoadModel is Model(WholeWorkbook), so apply_effects raises the flag — where before that branch :e other.improv left the document reporting clean. Clean was the wrong answer: LoadModel does not move model_state.file_path, so after it the in-memory document is a different one from the file the path names, and the bytes on disk and the bytes in memory really have parted company. That is all dirty claims.

It is worth a test rather than a comment because of what it gates: autosave_if_needed and the :q refusal both read this flag, so a user who loads and quits is now asked rather than silently dropping a document whose path is stale. :q! still discards. The path-staleness itself is pre-existing and is reported separately; this pins only that the flag follows from it.

a_view_only_batch_leaves_the_document_clean

The negative arm of improvise-gv9r's raise, and it must be aimed at apply_effects rather than at a Command: a view-slice command is folded straight into ViewState and never reaches this loop, so a negative written against the session door would pass however unconditional the raise was. It is a ViewOnly effect run through the loop that discriminates.

What it pins: a client's cursor, status line or panel does not put the document out of step with the bytes on disk, so it must not arm an unsaved-changes prompt or an autosave.

a_cell_edit_moves_only_the_record_it_touched

Tier 3 moves for the record written and for no other, and tier 2 stays put: a cell edit does not invalidate batches that merely share the document.

a_coordinate_rename_is_one_write_to_each_record_not_one_per_measure

The unit is the record, not the cell. Renaming a coordinate in wide records mode moves every measure cell in the row (gotcha 1), and that is ONE write to the record it left and ONE to the record it arrived at — not one per measure.

an_aborted_batch_bumps_nothing_it_did_not_run

Gotcha 21: a batch that aborts halfway must bump nothing after the abort — in either tier. apply_effects accumulates the Dirty inside the loop for exactly this reason, and the versions are read off the same place.

a_write_that_changes_nothing_moves_no_version

A write that lands the value already there invalidates nobody's basis, so it moves no version. This is a deliberate choice of semantics — observed change, not "a CellData effect ran" — and the safe direction, because the alternative rejects batches for nothing.

the_version_tiers_are_not_in_the_view_state

The counters are shared document state on App, never on the portable session slice (gotcha 30): a client inheriting another's idea of how far the document has moved is exactly the stale basis this machinery exists to catch.

a_page_flip_under_prune_empty_invalidates_the_cursor

improvise-7kvw, the case improvise-9mo9 decided the other nine and deliberately left open. SetPageSelection changes which records are visible and not which categories frame them, so a grid position normally keeps naming the same cell. Under View::prune_empty that is false: which rows survive pruning is a function of the page selection, so a page flip renumbers the grid while neither reset_cursor nor cursor_epoch moved — gotcha 24's failure in both directions at once, the host's own cursor stale and no client told.

9mo9 did not patch it because [/] paging is constant navigation and sending the cursor home on every flip is worse than the hole when prune_empty is off, which is the default. So the reset is conditional on prune_empty, and on the selection actually moving.

The fixture has three regions and two quarters whose non-empty rows differ, so position 0 names North on Q1 and South on Q2. The test asserts the row labels really differ before asserting the epoch moved; the negative arm asserts they really do not differ with pruning off, so neither case can pass vacuously.

Injected fault: drop the reset. The prune arm fails. Injected fault: make the reset unconditional. The no-prune arm fails.

a_page_flip_without_prune_empty_leaves_the_cursor_alone

The negative arm of the above, and it is load-bearing for the same reason gotcha 24 gives for SetNumberFormat: without it, "reset on every page flip" passes the positive test and costs the user their position on every [/].

selecting_the_page_already_selected_invalidates_no_cursor

The third arm, and the one the user actually hits. SetPageSelection resets the cursor only when the selection moved, and that guard shipped untested: deleting moved && left the whole workspace green at 1416, which review caught.

It is not an edge case. PageNext/PagePrev are odometer-style and emit a SetPageSelection for every page category on every keypress, so with more than one page category the non-carrying categories are always no-ops, and a single one-item category re-selects the same item every time. Unguarded, each such keystroke bumps cursor_epoch, sends the host's cursor to the origin and broadcasts Command::ResetCursor to every client on that view — improvise-75ld's "a no-op that refused everybody else", one tier down and aimed at the cursor instead of the basis.

The test asserts both preconditions before applying the effect — that the fixture really is on Q1, and that the grid really does prune — and that the row labels did not move before asserting the epoch stood still. So it cannot pass by re-selecting a page that was never selected, or by testing a grid that does not prune.

Both before placements are load-bearing, and both were wrong once. The page check used to run after the effect, which sets the page to Q1, so it could not see a fixture that started elsewhere: with the fixture moved to Q2 the test did fail, but at the row-labels assertion, under a message about renumbering. The sentence naming the property was not the assertion guarding it. The prune_empty check did not exist at all, and with pruning off the reset never fires whatever moved says — so paged_model(false) plus the deleted guard left the test green, proving nothing, with nothing saying so.

Both preconditions read App::view(), the view the effect writes through app.view_mut(), and not Workbook::active_view(), which is the document's default and is deliberately allowed to differ from it (gotcha 29). They agree in this single-client fixture, so the distinction is latent rather than a live bug — but an assertion about what an effect did must read the view that effect wrote.

Injected fault: delete moved &&. Fails by name, alone. Injected fault: paged_model(false) and delete moved &&. Fails on the prune_empty assertion; before that assertion existed, this combination passed. Injected fault: paged_model's fixture starts on Q2. Fails on the page-selection assertion, which now names it; before the move it failed on the row labels instead.

a_foreign_layout_is_not_shown_the_hosts_staged_drill_text

improvise-pubt, filed with an argument for unreachability that the test disproves. project::cell_display fills edit_text from App::display_value_in(layout, key), and since improvise-jytm that layout may be another client's. drill_state.pending_edits are keyed by (row index, RecordCol) against the host's drill layout (gotcha 1), and a row index in one layout names nothing in another.

The ticket read the arm as unreachable because "a foreign layout built by GridLayout::new carries no frozen records, so it should not produce a synthetic key". That is wrong: records mode auto-detects from the axes (gotcha 1), so an ordinary saved view with _Index on Row and _Dim on Column gives GridLayout::new a records layout whose cell_key returns exactly the synthetic (_Index, _Dim) key the drill arm matches on. The test fails on the original code, naming the host's staged text read out of a layout it was never staged against.

The fix is App::staged_drill_edit, which consults drill_state only when std::ptr::eq(layout, &self.layout). A pointer comparison rather than a name comparison because grid_in reuses &app.layout by reference where the view names agree and derives a fresh one otherwise, so identity is the question being asked. Whether the host's pending drill text should reach any other client at all is improvise-tom1 and is untouched here.

The positive arm — the host's own layout still shows STAGED — is in the same test, or "never consult the drill" passes, which would blank the TUI's own drill editor. The drill fixture needs a real RecordRow, not an empty Vec: the host's layout iterates the frozen snapshot, so an empty one gives a zero-row grid and the positive arm cannot be built.

a_default_that_names_nothing_says_which_invariant_broke

improvise-h074. App::resolve_view falls back to workbook.active_view for a name the document has not got and then indexes workbook.views[default]. Several of these call sites previously went through Workbook::active_view(), which carries .expect("active_view always names an existing view"); IndexMap's Index impl panics with no message at all, so a workbook whose default names nothing — reachable only by constructing the struct directly, since create_view/delete_view maintain it — failed with a bare index panic where it used to name the broken invariant.

Injected fault: restore &workbook.views[default]. The test fails on the expected substring.

---

ui/project.rs

a_window_reaching_the_last_row_carries_the_trailing_group_header

A collapsed group at the bottom of the grid has no items to hang off, and is still the only row that says those rows exist. A window that reaches the last row therefore carries it.

It must be seen; it is not yet actionable in a browser. web/app.js registers no listener on a group <th>, and toggle-group-under-cursor is still in the_command_names_that_still_need_the_host_are_these, so z resolves Upstream into Host::handle's empty vec — a black hole, not a slow path (doctrine(an_upstream_key_is_a_black_hole)). The TUI can expand it today; the browser can only show it.

Helper — app_with_a_trailing_group

Not a test — the fixture for that test and for its stops-short arm.

Region on Row with East in a group that comes last, so the group header is the final entry on the axis and nothing follows it. That is the shape axis_frame's trailing rule is about.

a_window_that_stops_short_does_not_carry_what_is_below_it

The other arm, and the one that would make a browser scrolled to the middle of a document draw the bottom of it: a window that stops short of the last row carries nothing below its last item.

a_window_opening_inside_a_group_names_only_the_nearest

A window opening inside a group still names it — otherwise a browser could not say which group a column or row belongs to — while body_from declines to paint a header that sits above the window.

And only the nearest one comes. The fixture puts two headers above the window (Hidden is collapsed, so it emits a header and no items, and Late's follows it immediately) because that is the only way to tell "the nearest header comes" from "every header above comes". With one group above, deleting the compaction leaves this green — it is a cost property, O(the groups a deep scroll crossed), and correctness alone cannot see it: group_of still finds the nearest and body_from still starts at the first item either way.

Helper — app_with_consecutive_headers

Not a test — the fixture for that test and for consecutive_headers_between_two_rows_all_come.

Two consecutive group headers between two rows: Hidden is collapsed, so it emits a header and no items, and Late's header follows it immediately.

consecutive_headers_between_two_rows_all_come

Headers between two in-window rows all come, not just the nearest.

Only the nearest wins group_of, so a compaction that kept one would still answer every question about membership correctly — and would silently lose the Hidden header, which is the only thing on screen that says those rows exist at all, and the only thing that will be clickable once the browser can toggle a group. It cannot today: z is Upstream and answered with nothing.

Above the window the rule is the opposite, and that arm is a_window_opening_inside_a_group_names_it_without_painting_it.

folding_what_the_server_emits_reproduces_what_the_server_has

The contract of the whole module: a client that folds what the server sends ends up holding what the server has. Not the fold axiom — the yardstick (server_truth) is built independently of the projections.

Helper — server_truth

Not a test — the yardstick that test and its siblings measure a fold against.

What the server believes the client should be holding, built straight from the authoritative state — the yardstick every fold is measured against, and built independently of the projections so the fold tests are not vacuous (doctrine(fold_tests_must_test_order_not_the_axiom)).

removing_a_category_empties_the_clients_formula_panel

Removing a category removes every formula that targets it — Model::remove_category ends with formulas.retain(|f| f.target_category != name). RemoveCategory is LayoutGeometry, and Dirty::of(LayoutGeometry) reported formulas: false, so the formula panel was never told: a client's FormulaCache kept showing a formula the document no longer has, indefinitely, with nothing to bring it back short of a reconnect. (Gotcha 35.)

a_late_client_converges_with_one_that_followed_from_the_start

Bootstrap is a compaction, not a replay: a client that connects late and folds only the seed lands exactly where one that followed the whole stream is.

the_projected_extent_is_the_whole_grid_not_the_window

The extent is the whole grid, not the window (improvise-bpq2).

A client resolving its own cursor keys clamps against this, so a producer that read it off the projected ranges would stop every browser cursor at the bottom of the screen — and would look right in the TUI, whose cache covers the whole grid at offsets zero (gotcha 44). The fixture is deliberately not square (3 rows, 2 columns) and the window is smaller in both directions, so it separates "the whole grid" from "the window" and from "rows and cols swapped".

a_reshaped_grid_folds_to_the_desired_state

Reshaping is the case the ordering exists for. Growing is the half that bites: cells at positions the client does not have yet are dropped, so the labels that create those positions must go first. Shrinking is here for the other direction — cells that no longer fit must not be sent into a grid that has already been cut down.

a_renamed_column_whose_total_did_not_change_still_gets_its_total_back

improvise-hkt6, found by Copilot on PR 20. The diff must carry the mirror to desired, and it stopped doing so for one shape: a column renamed while its total stayed the same number.

apply blanks the total under a relabelled column (gotcha 33(b)), so the client's total went blank; but the diff compared mirror.totals against desired.totals, found them equal, and sent no Totals. ClientSession::project_diff then assigns self.grid = desired, so the server believed the number was still on screen — a cache wrong forever, with nothing in the stream to correct it, which is precisely the direction gotcha 35 keeps Dirty::of sloppy to avoid.

The assertion is the one a_reshaped_grid_folds_to_the_desired_state makes — fold the updates onto the mirror and require the result to be desired — because "a Totals update was emitted" would pass for a producer that emitted the wrong totals.

The fixture renames the middle column and keeps every total equal, so it separates "re-sends what the relabel blanked" from "re-sends whenever the labels move at all".

renaming_an_axis_category_resends_every_cell

The producer half of the category-rename rule, and the reason the apply half is not cosmetic: the two compose into "wrong forever" without it.

grid_updates diffs the cells against the framed mirror — the mirror with the frame updates already folded in. So an apply that kept the cells through a cats change made the producer agree they matched and send nothing, ClientSession::project_diff then assigned self.grid = desired, and the client held cells answering the old coordinates with no update in the stream that would ever correct them.

An axis swap is the case that bites: two categories can legally share an item name, so Region = [North] becoming Department = [North] leaves the labels identical and the CellIds valid and pointing at the other record.

a_session_that_switches_views_is_projected_the_view_it_switched_to

The original bug: ClientSession::apply folded straight into the session's mirror, so a Command::SwitchView moved the mirrored active_view while the workbook's stayed put (gotcha 29). viewport() then read the cursor under the new view's key and the next project() built the grid from the other view.

improvise-jytm answers it the other way round — the mirror is now authoritative and grid_in is built from it — so the assertions moved with it: the asker's mirror names "Second", the document's default is still "Default", and the rows it is sent are the ones "Second" has. The fixture's two views are transposes of each other, so "was I sent the right grid" cannot be answered by accident.

Two faults injected 2026-09-20, both of which fail it. Pointing grid_in at app.view_name() regardless of the session reproduces the original defect exactly. Removing ClientSession's projected_view observation is the new way to reproduce it: a switch is ViewOnly, so nothing in Dirty carries it and the session must notice its own move or never rebuild its grid at all.

Helper — app_with_two_views

Not a test — the fixture for every lockstep and rebase test in this file.

A second view whose axes are the first one's transposed, so the two grids cannot be confused for each other: "Default" rows are regions, "Second" rows are measures.

a_view_switch_moves_only_the_switchers_mirror

The successor to a_view_switch_moves_every_sessions_mirror_not_just_the_switchers, which pinned the lockstep improvise-jytm removes. The model is shared and the UI is not, so a bystander keeps its view, its scroll and its grid.

Three assertions, because the old bug can come back through three different doors: the bystander's mirrored active_view, its row_offset (nonzero, so a mirror reset shows as a move rather than coinciding with the origin), and whether project_all names it at all. That third one is the sharpest — a bystander repainted for somebody else's switch is invisible in the first two, since it would be repainted with its own view's content and look right.

The scroll is drained through project_all before the switch, so the fan-out the test reads names only what the switch caused.

Fault injected 2026-09-20: restoring the adopt_active_view loop (push the accepted view into every mirror) fails it, as does removing the projected_view observation.

a_refused_switch_corrects_the_asker_and_nobody_else

improvise-6c6q. apply_switch relayed Broadcast::ToOthers, so on the refused path every client but the one that was wrong was told the accepted name. The asker's own slice kept the name it had folded optimistically while its server-side mirror held the accepted one — gotcha 24's mismatch, arriving through the asker's own keystroke. apply_switch's doc comment claimed the hole was closed, which is worse than the hole.

The test drives the asker's own ViewState, folding the command the way a real client would before handing it to the door. That is the whole cost of the ticket: the sibling bug in apply_model shipped green because its test read a "what do the others hear" helper, which looks away from the asker by construction. There is deliberately no such helper in this test.

The bystander is asserted from a view it can be moved OFF. A first version left it on the default, where a wrongly-addressed relay folds to a no-op — so the assertion read as coverage and was vacuous, which review demonstrated by injecting ToAll and watching this test pass. It now sits on "Second", and the audience itself is asserted (Broadcast::To(asker, …)) rather than inferred from what somebody's slice folded to: the audience is the message (gotcha 36), so it is the thing to read.

Two faults injected 2026-09-20, both of which fail it: relaying ToOthers (the original improvise-6c6q bug) and relaying ToAll (the over-correction). The second also fails folding_the_relay_puts_a_refused_asker_back, which reads frames rather than folded state.

deleting_the_view_this_client_is_on_moves_it_to_the_default

The host-side half of deleting_a_view_moves_the_client_that_was_on_it_and_tells_it, and a desync that predates improvise-jytm: Workbook::delete_view moved the document's active view and nothing moved ViewState::active_view, so the TUI's cursor was read under a key naming a view that was gone. It was invisible while draw_status read the workbook's copy; improvise-k69y makes it read the client's, which is what turns it into a wrong badge.

The negative arm is the discriminating one: deleting a different view must leave this client where it is. Without it, unconditionally seating the client on the default would pass the positive assertion. The fixture therefore carries three views, and the first delete is of the one nobody is on.

Fault injected 2026-09-20: dropping the view_state.active_view line from effect::DeleteView fails it.

deleting_a_view_moves_the_client_that_was_on_it_and_tells_it

A hole improvise-jytm opens and must therefore close. Before the split, Workbook::delete_view moved the document's active view and adopt_active_view pushed the result into every mirror, so deleting the view a client was on moved that client. The split removes both, and without adopt_deleted_views the client is left naming a view the document has not got: App::resolve_view falls back to the default, so it is projected the default's grid while its slice says otherwise, and viewport() reads its cursor under a key nothing will ever write again.

The fixture carries three views and the bystander sits on the one the delete does not remove. That is the whole discrimination: a first version left the bystander on the default, where a relay addressed to everybody folds to a no-op, and review demonstrated the vacuity by replacing Broadcast::To with ToAll and watching the test pass. The evidence entry had even stated the risk in one sentence and claimed the assertion covered it in the next — the exact shape this file exists to stop.

So four assertions, over three different surfaces. The audience is read off the Broadcast itself, because the audience is the message (gotcha 36). The client that lost its view moved, in its mirror and in its own slice, the latter driven by folding what the host relayed. And the bystander is still on "Third" — in its slice and in its mirror — which only means something because there is somewhere for a wrong relay to have moved it from.

The deletion itself is asserted, because Workbook::delete_view refuses to remove the last view: a fixture with one view would pass every other line here while deleting nothing.

Two faults injected 2026-09-20, both of which fail it: dropping the adopt_deleted_views call from apply_model, and addressing its relay ToAll.

a_session_on_another_view_is_projected_that_views_axes

The hole review found in improvise-jytm's first cut: only the grid half of "every projection follows the session's view" was checked. category_tree_in and tile_bar_in are reached through ClientSession::view_in, and replacing that function's body with app.view() — the host's view — left the whole suite green.

The harm is worse than a wrong pixel. A category's axis is what the tile bar is for: it is the control a user reassigns axes with. Under the fault a client on another view sees the wrong axis for every category, in the widget it would use to change them, while the grid beside it is correct — nothing to notice and nothing that fails.

app_with_two_views is what makes two assertions enough: "Second" is "Default" transposed, so reading the wrong view swaps Row and Column rather than blurring them, and the test names both categories rather than one.

Fault injected 2026-09-20: ClientSession::view_in returning app.view() fails it.

walking_the_view_history_moves_no_document_state

The other hole review found: of the three effects improvise-jytm reclassified to ViewOnly, only SwitchView had a guard (a_wire_view_switch_leaves_the_document_clean). Putting ViewBack or ViewForward back to Model(LayoutGeometry) left all 1313 tests green — which is gotcha 3's whole point: a mis-declared kind is silent.

It asserts the consequences rather than the declaration, because those are what a reader can price. App::apply_effects raises model_state.dirty for any Model(_), and the browser worker persists on that flag — so a mis-declared view-back rewrites the entire OPFS document (gotcha 41: Storage is whole-document) for a move that changed nothing. And it bumps layout_version, so every other client's next Command::Model is refused LayoutMoved (ADR 001) because somebody pressed a navigation key.

The assert_ne! on the active view before each pair of assertions is the check-the-checker: an empty history stack makes ViewBack a no-op, and a no-op dirties nothing whatever its kind.

Both effects are driven, because they are separate kind() implementations and review injected them separately.

a_fresh_session_opens_the_documents_default_view

The other half of improvise-jytm's contract, and the reason Workbook::active_view survives at all: it is what a freshly connected client opens. The fixture switches an existing session first, so a connect that read any client's view rather than the document's would be visible.

Fault injected 2026-09-20: pushing the accepted view into every mirror in apply_switch moves the document's default nowhere, but making apply_switch write workbook.active_view fails it.

Helper — diff

Not a test — the accessor every diff-subject test in this file reads its answer through.

The two halves of an Applied::Diff, for the tests whose subject is not the rebase. It panics rather than defaulting, so a command that starts replacing the document is a loud failure here and not a silent Dirty::NOTHING.

Note: before the comment-cap wave this helper's doc comment was preceded by an orphaned paragraph ("One connected client, as its host really drives it…") describing a type that no longer exists at that site. Dropped.

Both a_model_command_that_moves_the_active_view_binds_every_mirror and a_model_commands_view_switch_reaches_the_asker_too were retired by improvise-jytm, along with the to_others helper whose blind spot the second one existed to cover. No ModelCommand moves anybody's active view any more: ViewBack/ViewForward left the wire because the stack they pop is one client's history, and a document swap lands in the rebase arm. The property the second test defended — a correction must reach the client that folded it — is now a_refused_switch_corrects_the_asker_and_nobody_else, which drives the asker's own slice rather than a helper.

a_geometry_command_invalidates_every_clients_cursor_including_the_askers

TransposeAxes and CycleAxis call app.reset_cursor() because renumbering every row and column makes the old indices name different cells (gotcha 24) — but that reset only reached the authoritative App's own view slice. Each ClientSession keeps its own cursor in its mirrored ViewState, and nothing reset those, so every client was left indexing a grid that had changed shape under it: Viewport::window clips, so what a bystander actually saw was a blank grid, until it happened to scroll (improvise-gvnb).

The asker is checked alongside the bystander, and that is not symmetry for its own sake. A Command::Model is the one kind a client cannot fold optimistically, so the asker is in exactly the same position as everyone else, which is why the reset is Broadcast::ToAll and not the To(id, …) a refused SwitchView correction is.

a_model_command_that_does_not_reshape_the_grid_leaves_cursors_alone

The other arm. A model command that leaves the grid's shape alone must not throw anyone's cursor away: where a client is looking is its own UI, and only a change to the shared geometry may invalidate it. Without this, "reset on every model command" would pass the test above.

a_bystanders_own_slice_follows_the_invalidation

And the bystander really is shown the new grid from the origin, rather than a window clipped to nothing. This is the half the mirror reset alone cannot give: the client's own slice is driven by what the host relays, so the test drives it the way Host::command does.

Helper — host_dispatch

Not a test — the relay stand-in that test and its siblings drive a client's own slice with.

A stand-in for Host::command's relay, driving one client's own slice. to is the client being driven and from the one that asked, because that pair is exactly what Broadcast's audience is about.

a_model_command_that_leaves_the_active_view_alone_broadcasts_no_switch

Renamed in review: it was ..._broadcasts_nothing, and since improvise-9mo9 it asserts a ResetCursor broadcast, so the name read as a lie about its own assertion. No client folded an active view, so there is no SwitchView correction to make. What the others are told is the Dirty it produced — and, since improvise-9mo9, the ResetCursor an AddCategory owes, because a new category lands on an axis in every view. The assertion is the exact broadcast list rather than "contains no SwitchView": a weaker form would go on passing if the cursor invalidation were dropped again.

every_reshaping_model_command_invalidates_every_cursor

Only TransposeAxes and CycleAxis reset the cursor, while SetAxis, HideItem, ShowItem, ToggleGroup, TogglePruneEmpty, AddCategory, RemoveCategory, AddItem, AddItemInGroup and RemoveItem renumbered the grid and did not (improvise-9mo9). SetAxis is the case that shows the shape of the bug: CycleAxis is SetAxis to Axis::next(), so a pair differing only by an argument disagreed. Because reset_cursor is also the only writer of cursor_epoch, every one of these left the host's own cursor stale and broadcast nothing, which gotcha 24 records as strictly worse than doing neither — the diff then believes each client already holds origin-positioned content and sends nothing, so they stay stale permanently rather than until the next scroll.

Two faults were injected. Deleting app.reset_cursor() from SetAxis fails the bystander-cursor assertion (left: 1, right: 0). Changing the fixture so the item ShowItem reveals was never hidden fails the first assertion — "ShowItem reshaped nothing, so this case is not evidence of anything" — which is the one that stops this from becoming a table of commands asserted against a grid that did not move. That guard matters most for AddItem: the fixture puts two categories on Row precisely so an append to the inner one inserts a row inside every outer block, because on a single-level axis an append lands at the end and moves nothing, and a one-level fixture would have made the reset look unearned.

Helper — app_with_a_reshapeable_grid

Not a test. Two categories on Row, one of them grouped, one item hidden and one item left with no data — so ShowItem has something to reveal, ToggleGroup has a group to fold and TogglePruneEmpty has an empty row to prune. Every command in the table has something real to reshape, which is what the first assertion checks.

drilling_resets_only_the_drill_views_cursor

improvise-9mo9 made SetAxis call app.reset_cursor(), and DrillIntoCell issues eight or so SetAxis effects to build the _Drill view. The PR claimed that was safe and did not test it — the claim was that SwitchView precedes the batch, so reset_cursor writes the active view's cursor, which by then is the freshly created _Drill one, while the view drilled from keeps its entry in ViewState::cursors. That is an ordering argument about effects inside a batch, which is exactly the class of claim gotcha 23 exists because people get wrong.

Two faults injected. Removing the SwitchView from DrillIntoCell's batch fails the first assertion — "the drill did not switch views, so this proves nothing about per-view cursors" — which is the guard that stops the test passing for the wrong reason if the drill ever stops switching. Changing reset_cursor to clear the whole cursors map rather than the active entry fails the third: left: (0, 0), right: (1, 1), the cursor in the origin view thrown away.

It asserts both sides on purpose. That the _Drill view opens at its origin is half the property; that the other view keeps (1, 1) and row_offset: 1 is the half that would have broken, and a test asserting only the first would pass against a map that had been emptied.

an_accepted_no_op_invalidates_no_cursor

The third negative arm, found in review. Gating the reset on Ok is not the same as gating it on "something changed", and six commands the document accepts changed nothing and reset every cursor anyway: SetAxis to the axis already held, HideItem on an already-hidden item, ShowItem on a never-hidden one, AddItem of an existing item (Category::add_item dedupes and returns Ok), and RemoveItem/RemoveCategory of something absent.

The first assertion is the non-vacuity guard and it is doing real work here: it uses grid_shape to prove each case really is a no-op, so a fixture drifting into one where the command does reshape fails rather than passes.

Review then found five more cases the table could not reach by construction, because app_with_a_reshapeable_grid had no formula, no category on Axis::None and no unknown-name case — the one-category-fixture shape the doctrine names. The fixture now carries a Profit formula, a Channel category parked on Axis::None, and a Ghost entry hidden under a name the model never had, and the table covers: SetAxis on an unregistered category, HideItem/ShowItem on a name the grid never drew, ToggleGroup on a group that does not exist, and AddItem { _Measure, Profit }.

That last one is the one to remember. The guard it replaced read category.items, but _Measure's effective items are explicit ∪ formula targets (gotcha 7), so it called a drawn column "new". Two of the five injections initially failed to reproduce, and both times the injection was wrong rather than the guard: the ShowItem case needed a hidden ghost name, since is_hidden is false for a name nobody hid; and the AddItem case had to compute its predicate before the add, because afterwards the item really is in category.items. An injection that does not reproduce the original ordering tests nothing — the same shape as the simulated-fix confirmation in gotcha 48.

ToggleGroup's first guard was also wrong and this test caught it: add_item_in_group sets item.group without pushing to Category::groups, so asking groups said "no such group" about the fixture's real one.

Faults injected, two, both caught by name: removing SetAxis's moved check, and removing AddItem's fresh check. The TUI repro needs no test harness — TileSelect's r emits SetAxis with no comparison against the current axis.

naming_a_measure_at_the_bottom_right_leaves_the_cursor_put

A regression this PR introduced and review caught. AddItem gained a reset_cursor, and CommitAndAdvance{Down} on the trailing + column routes RecordCol::NewMeasure through add_measure → effect::AddItem. inserts_a_column matches only NewCategory, so NewMeasure falls through to the advance — and EnterAdvance at the bottom-right returns AbortChain, emitting no SetSelected. Nothing restored the cursor. The SetSelected repair added for +cat covered that arm only.

The fix moves the restore before the advance, which is gotcha 21's rule (tidy-up goes where an abort cannot reach it) applied to the cursor rather than to the mode. Injecting the old ordering fails it: left: (0, 0), right: (0, 3).

a_refused_model_command_invalidates_no_cursor

The second negative arm, and the one the first version of improvise-9mo9's fix got wrong. It was a loop of one until review: it named a class and tested AddItem alone, and injecting reset_cursor() into AddCategory's Err arm — the very bug it was written for, one effect over — left it green. It is now a table over all three refusable effects, and that injection fails it by name. AddItem and AddItemInGroup report a failure — an unknown category — through status_msg, and the reset was written after the if let Err, so a refused add bumped cursor_epoch and broadcast Command::ResetCursor to every client for a change the document never made. AddCategory had been gated on success in the same commit, which is what made the inconsistency visible.

SetNumberFormat's arm cannot see this: it tests a command that succeeds and reshapes nothing, where this tests one that reshapes nothing because it was refused. Injecting the fault — moving the reset back outside the match — fails it on the bystander's cursor. The first assertion is the non-vacuity guard: if the category ever became valid, the case would stop being about a refusal and the test would say so rather than pass.

Helper — grid_shape

Not a test. The row_items/col_items of the layout, as text: which cell each position names. It is compared before and after rather than counted, because a cats change can re-identify every position without changing how many there are.

a_clients_view_only_command_moves_only_that_clients_mirror

The other half of the door: a command that is nobody else's business must stay that way. A client's scroll must not move the authoritative App's own view slice — the model is shared, the UI is not.

a_document_swap_reseeds_every_session

Replacing the document must clear the cursor map, because the incoming document's view names have nothing to do with the outgoing one's (gotcha 24). install_workbook does that for the App's own view slice, but nothing touched the session mirrors: a connected client went on naming the old document's view and scrolling by the old document's cursor, so it was projected a window past the end of a document it had never seen — an empty grid, with nothing that would ever bring it back.

Helper — other_document

Not a test — that test's incoming document, and the one whose rename hides a bug.

A different document, with a differently-named active view, so a mirror still keyed to the outgoing one is visible rather than coincidentally right.

rebase_hands_every_session_a_snapshot_of_the_new_document

The reseed is reached from the projection path itself, not from a hook a host must remember to call — Sessions is not reachable from the effect that installs a workbook, so a convention there would be unenforceable. rebase exists for the other half: the client's own view slice lives on the client, and only a fresh Snapshot resets it.

a_document_swap_over_the_wire_rebases_every_client

A document swap asked for by one client must reach every other client's own view slice, and the host must not be able to miss it.

Command::Model put the WholeWorkbook mutations on the wire (improvise-kmgs), so LoadModel is now something another client can ask for. The server stayed honest — the generation bump reseeds every mirror on the next project — but a bystander went on holding cursors["Default"] = row_offset 3 from a document that no longer exists, and Applied { dirty, broadcast } said nothing that would tell its host to send a Snapshot. Sessions::rebase, the only thing that produces one, had no non-test caller in the tree.

That is gotcha 24's cursor-vs-geometry mismatch, arriving through another client's command — the same shape doctrine(lockstep_binds_every_session_not_the_asker) names, one layer out: the obligation was addressed to a host in prose instead of being carried by the value it gets back.

The server's own mirror self-heals off the generation bump (a_document_swap_reseeds_every_session covers that half); this is the other one, the client's slice, which only a Snapshot resets.

Helper — a_document_shaped_differently_under_the_same_view_name

Not a test — the fixture that test needs instead of other_document.

A different document whose active view has the same name as the outgoing one, and a different shape: one row where the outgoing document had three.

other_document renames the active view, and that rename is what makes the bug below invisible: apply_model broadcasts a SwitchView when the name moves, a client folds it, and the stale cursor is then read under a key nothing has written yet — right by accident. Same name, different geometry is the case with nothing to hide behind, and it is the ordinary one: two revisions of a document both call their first view Default. (Gotcha 39's fixture warning.)

Helper — app_over_a_loadable_document

Not a test — the App that makes a wire LoadModel in that test really replace the document.

An App whose storage already holds that document, so a Model(LoadModel) arriving over the wire really does replace the document rather than setting an error status.

a_command_that_leaves_the_document_in_place_is_not_a_rebase

The other arm of the same rule, so the check is not one that always says "rebase" (doctrine(one_arm_of_a_symmetric_rule_is_evidence_about_that_arm_only)). A model command that mutates the document in place, and a view-only one, are both diffs — the question apply_model asks is whether the generation moved, not which variant arrived.

ui::project::tests::a_load_that_fails_is_not_a_rebase

Qualified because ui::host::tests has a test of the same name: this is the apply_model one, and a bare leaf was satisfiable by either.

A LoadModel naming a document the storage does not hold leaves the document alone, so it is a diff too. This is why apply_model asks the generation rather than the variant: "is this one of the three WholeWorkbook commands" is not the same question as "was the document replaced", and a client would be reseeded onto the document it was already holding, losing its cursors to a failed load.

Note — the cell_identity module

Emitting and resolving the surrogate a client names a cell by (ADR 002). The two halves are tested together on purpose: an id that is emitted and never resolved, or resolved by re-deriving it the same way it was emitted, would agree with itself while agreeing with nothing else.

the_two_measures_of_a_row_share_a_record_and_differ_by_measure

Gotcha 1's unit, projected: two measure columns of one row are one record and two cells. Both halves matter — a surrogate that also varied by measure would break record_versions, and one that did not vary by measure at all would make the two columns the same cell.

a_projected_record_id_resolves_to_the_cell_it_was_projected_for

The round trip that makes the surrogate usable: what the server projected for a position resolves back to the key that position has. Every row is checked, because one row cannot tell "resolves correctly" from "resolves to the first record".

Helper — raw_app

Not a test — the fixture for every stored-cell surrogate test in cell_identity.

Region on Row, _Measure on Column. Nothing regular sits on None, so every cell is a stored cell. Three rows and two measure columns, so "the record" cannot be confused with "the row" or with "the cell".

a_projected_aggregate_id_resolves_to_the_same_constraint

An aggregate names a coordinate constraint, and it must come back as the same constraint — including the page and filter coordinates baked into the layout's key (gotcha 27), which is why this resolves against cell_key rather than against the row and column labels.

Helper — aggregated_app

Not a test — the fixture for the aggregate half of cell_identity.

The same, with a second regular category left on None, so every visible cell sums over Product rather than being one stored cell. That is the fork EditOrDrill makes, and the one the wire projects.

a_surrogate_for_a_removed_row_fails_to_resolve_and_the_others_do_not

The first arm of staleness. The identifiers are held constant and only the thing that makes the id stale is varied (doctrine(a_rename_can_hide_a_stale_key)): "East" is removed, so its surrogate must fail to resolve — while "North"'s, taken from the same projection, still resolves to the same cell. Without that second assertion a resolve_cell that returned None for everything would pass.

an_empty_cell_can_be_written_through_its_projected_id

improvise-1ey is "type a number into a cell and see the value update", and the cell a user types into is usually empty — so it is named by the grid and by nothing in the store. The index cannot answer it, which is why resolution falls back to the layout, and this is the case that makes that fallback load-bearing rather than defensive.

a_write_by_surrogate_lands_on_exactly_that_cell

Two rows and two measure columns, so a write that went to "the first cell", "the whole row" or "the whole column" is visible.

a_write_through_a_dead_surrogate_changes_nothing

The second half of "reject, nothing to write to": a surrogate that does not resolve must apply nothing, not fall back to some other cell. The document is compared whole, because a write to the wrong place is exactly the failure being excluded.

It goes through model_effects rather than the wire form, because the session door reaches the same conclusion first and typed — Checked::against refuses this command with Rejection::RecordGone, so a Checked over it does not exist. What is asserted here is the floor underneath that door: the key path and any host resolving a command locally get an empty batch rather than a misdirected write.

a_records_mode_cell_is_named_by_its_coordinates_not_by_its_position

The trap gotcha 1 sets. GridLayout::cell_key returns a synthetic (_Index, _Dim) key here, carrying positions — the very thing ADR 002 rejected _Index for, because position names a different record after a filter, an insert or a delete. The projected id must come from RecordRow::coords instead, which is what resolving it back to the real cell proves.

Helper — records_app

Not a test — the fixture for every records-mode surrogate test.

Wide format: one row per coordinate tuple, a column per raw measure, the coordinate columns in front and a + column behind. Two records and two measures, so neither "the row" nor "the column" can stand in for the record by accident.

the_measure_columns_of_a_record_row_share_its_record

Two measure columns of one row are two cells of one record, and the next row is a different record — the same shape the pivot case has, checked here because records mode reaches it by a different path.

a_coordinate_column_is_projected_with_no_identity

A coordinate column is not a cell to write into: editing it renames that coordinate across every measure cell in the row (gotcha 1), which is a different intent from SetCell. So it is projected with no id at all, and a client cannot compose a write against it.

a_write_moves_the_version_under_the_clients_own_surrogate

The version counter is keyed by the same surrogate the client holds, which is what lets one identity answer both ADR 002's question and ADR 001's. A write must move the counter the client would check.

Helper — apply_caught_up

Not a test — and note it is #[cfg(test)] fn, not #[test] fn.

Sessions::apply for a client that is fully caught up, for the tests whose subject is not the basis. The staleness tests build the Basis by hand, so neither kind can pass by accident through the other's convenience.

---

Note — the basis_check module

ADR 001's basis check: a batch composed against a picture of the document that has since moved is refused, and one composed against the current picture is not (improvise-xwi6, improvise-fkfg).

Both arms, per tier. A check nobody has watched reject is indistinguishable from one that always accepts, and a check nobody has watched accept is indistinguishable from one that always rejects (doctrine(check_the_checker)). So every tier appears twice.

The fixtures hold every identifier constant and vary only what makes the basis stale (doctrine(a_rename_can_hide_a_stale_key)): the record a batch names is projected once and reused, so a rejection can never be an artifact of the test naming something different the second time.

a_cell_edit_is_rejected_when_the_layout_moved_under_it

The class-2 case, and the one test to keep if only one survives.

A CellData batch is stale when the layout moved, even though no record it names was written: the coordinates its key hangs off changed underneath it. This is A removing an item while B commits an edit keyed by it, and it is corruption rather than a lost keystroke — which is exactly what a cell-level lock would not have covered, because holding a cell does not stop anyone deleting the row.

Helper — basis_for

Not a test — the claim every basis_check test moves exactly one tier of.

The basis a caught-up client holds for one record. Built by hand rather than through caught_up so each test can move exactly one tier of it and leave the rest true.

a_cell_edit_composed_after_the_reshape_is_applied

The other arm of the same tier. A batch composed after the reshape is applied — so the rule is "the layout moved since you looked", not "the layout has ever moved".

a_write_is_rejected_when_the_record_it_names_moved

Class 1: somebody wrote the record since this client read it. Data staleness — the record still resolves, so force is coherent and the reason says so. The identifiers do not move, only the value, so a rejection cannot be a renamed key in disguise.

a_write_survives_another_record_moving

The arm that keeps the tier from being "reject whenever anything was written". A record moving does not invalidate a batch that names a different one — which is the whole reason the tier is per-record rather than a single counter.

a_write_to_a_record_that_no_longer_exists_says_so

Class 2 by the other route, and the reason ADR 002 put a maintained index in DataStore rather than regenerating one from the layout: "the record is gone" and "the record changed" must not collapse into one answer. There is nothing to write to here, so nothing to force — force never creates structure.

The test brings the layout claim up to date so that, whatever kind RemoveItem wears, only the record tier is left to answer. It is CellData today — removing an item drops its cells and moves no view's axes — so that line is currently a no-op; it is kept because a reclassification to LayoutGeometry would otherwise reject on the tier above and hide the case this test is about, silently turning it into a second copy of a_cell_edit_is_rejected_when_the_layout_moved_under_it.

a_claim_that_omits_the_record_it_writes_is_not_a_free_pass

The force-by-omission hole, closed. Which records are checked is the server's decision, read off the command, so a client that simply left its record out of the claim does not skip the tier — it is told it never stated one. Without this, ADR 001's "force is a re-composition, not a bypass" would have a bypass sitting in the message format.

a_document_swap_invalidates_every_claim

The top tier invalidates everything, whatever the other two say. It is checked first so the answer names the largest thing that moved: a client told "a record moved" after a document swap would go looking for a record that is not there.

a_rejected_batch_changes_nothing_anywhere

Rejection is a normal outcome: nothing is applied, nobody else hears about it, and no basis moves. A rejection that dirtied something would tell every other client to redraw for a change that did not happen.

a_published_basis_follows_the_grid_updates_it_describes

The order ClientSession::basis_update claims is load-bearing, pinned.

The basis names record counters for the cells the grid updates in the same batch carry, so a Projection::Basis that overtook them would let a client claim a version for a value it had not been shown — the mirror image of gotcha 33's rule that labels precede the cells they frame, and unlike that rule it had nothing holding it but the order of two statements in project_diff. Proved by injecting the swap (gotcha 43).

a_client_that_folds_the_stream_composes_an_accepted_batch

The check is only satisfiable because the server publishes the basis — which ADR 001 does not mention, and without which a client could only forge one. So: a client that folds what it is sent can compose a batch that is accepted, end to end, with no test-only constructor anywhere in the path.

re_composing_against_the_republished_basis_is_accepted

And the other side of publication: after somebody else writes, the stream carries a new basis, so the client that folds it can re-compose and be accepted. That is ADR 001's force — a re-composition against the current basis — reachable without any mechanism that skips the check.

ui/host.rs

connecting_hands_that_client_a_snapshot_that_draws_the_document

AC 2: the bootstrap is sent on connect, addressed to the new client, and it is not empty — a Snapshot whose seed carried nothing would satisfy "a snapshot was sent" while leaving the client blank.

the_reported_extent_bounds_the_snapshot

The extent is what the snapshot is built at, so a client that says it can see one row is not sent two. This is the property that makes Viewport the handshake rather than an afterthought.

without_the_command_the_cell_is_empty

doctrine(check_the_checker): the preceding assertion would also pass if the seed happened to contain the value already. Inject the fault — never send the command — and it must fail.

a_bystander_is_projected_another_clients_edit

A cell edit is the whole document's, so a client that did not ask for it is told about it too. The projection is per-session because the viewports differ, not because the change does.

Helper — model

Not a test — and note it is #[cfg(test)] fn, not #[test] fn.

A model command from a client that is fully caught up, for the tests whose subject is not ADR 001's basis. It goes through handle like any other frame; only the basis is supplied rather than folded, and caught_up can do that because it holds the App (a client cannot). The staleness tests build their own Basis.

Helper — addressed

Not a test — how every host.rs test names a cell the way a client would.

The same cell as a client names it: the opaque surrogate the server projected on CellDisplay::id, which is the only handle a client has (ADR 002). A CellKey is not constructible over there.

a_scroll_reaches_only_the_client_that_scrolled

A view-only command reaches nobody else. The counterpart to the test above: if every command fanned out, that one would pass for the wrong reason.

a_batch_of_commands_is_applied_whole_and_in_order

One keypress resolved on a client is up to three commands, and the host must take all of them (improvise-bpq2).

The fixture is the shape that actually arrives: a cursor moved past the bottom of a one-row window, so the selection and the offset that follows it travel together. The assertion is the offset, not the selection, because a host that applied only the first command would still look right on selected — the window is what goes wrong, and it goes wrong silently.

a_batch_is_abandoned_at_the_first_rejection

A refused batch applies nothing after the refusal. The stronger half — nothing before it either — is a_refused_batch_leaves_the_commands_before_it_unapplied; this is kept because the two halves fail independently.

a_refused_batch_leaves_the_commands_before_it_unapplied

improvise-955n / improvise-08wb. The bug: ADR 001 is emphatic that a stale batch is never applied, not even partially, and improvise-ondj went to the trouble of a type (Checked) to make the unchecked path unconstructible. But Checked is per command, and a ClientFrame::Commands is a sequence of them: Host::commands applied in order and abandoned at the first rejection, so a frame whose second command was refused had already applied its first. The guarantee was true of a single command and false of a batch.

The earlier command is a view command, which is not a concession: it is the shape the one real producer emits (keys::commit — one mutation among view commands), and it is still observable, because the host folds it into that client's mirror. A frame may carry more than one mutation; what it may not do is apply some of them and refuse the rest, which is clause 4 and open.

A refusal test needs an accepting one (doctrine(a_helper_that_builds_the_world_the_code_wants_tests_nothing): pair a refusal with an acceptance, or the refusal can pass by refusing everything). The second half runs the same leading SetSelected followed by a command whose basis is current, and requires both to land. Without it, a SetSelected that simply never worked would clear the first assertion and prove nothing at all.

Inside it, the stale claim is built from caught_up and then has a single tier bumped, so the refusal can only be about the tier that was moved — a hand-built basis that is stale in three ways at once cannot tell which check answered.

a_later_write_in_a_chain_still_answers_for_its_own_record

improvise-955n. The regression 90ea26e introduced, and the reason clause 2 cannot be applied to all three tiers.

Hoisting the basis check to the frame hoisted all of it, so every mutation after the first was admitted with no record check at all. Two clients, one record: A composes [SetCell(North), SetCell(South)] against an honest current basis, B writes South, A's frame arrives — and A's write to South landed on top of B's silently, which is exactly the lost update ADR 001's record tier exists to prevent. At the base commit the second command was checked and the frame refused.

The per-command check was doing two jobs and the commit noticed one. "Is the client's picture of the document current" is a frame-level fact and hoists correctly. "Is the client's picture of this record current" is not: which records are at stake is a property of the command, which is why records_named is an exhaustive match over ModelCommand (gotcha 43). The incapability argument that motivated clause 3 is true of records a frame creates and was over-generalised to records it merely writes — and those are the ones the tier is for.

Three arms, because a fix that refuses everything would pass the first alone and a fix that checks nothing would pass the last two.

In arm 2, B moves South with a SetCell, which is CellData: neither the document nor the layout tier moves, so only the record tier can discriminate — which is what makes this a test of the record tier and not of the hoist in general.

Arm 3 — the same record twice in one chain. Legitimate and likely (two measures on one record is one record) and it works for a reason worth pinning rather than rediscovering: every check happens before any command is applied, so the second write is compared to the record's pre-frame counter, not to the one its own sibling just bumped. Were the checks interleaved with the applies, this frame would refuse itself with RecordMoved.

Read this before closing clause 4. Interleaving is tempting there, because it would close the silence: [RemoveItem(North), SetCell(North)] under interleaving is refused RecordGone instead of silently half-applying, which is half of what clause 4 owes. So check-before-apply is not a free win — it buys the legitimate double-write and sells the loud failure on a broken chain, and clause 4 has to pay that back some other way. Measured rather than argued: injecting the interleaving fails exactly two tests, this one and a_frame_whose_later_mutation_fails_is_not_yet_undone, and the second failing is the trade. Arm 3 is what stops someone taking the trade without noticing they made it.

a_data_dependent_chain_lands_in_order

The capability the contract is for: a chain of data-dependent commands, applied in order, each seeing what the previous one left.

[AddItem("East"), SetCell(the East record)] is the archetype, and it is the case a per-command basis check cannot admit however it is ordered: a client cannot claim a counter for a record its own chain is about to create, because BasisCache::claim filters to records the cache has been told about. So the basis is answered once, on the first command that carries one, and the rest of the chain runs against what its predecessors left.

The second assertion is the one that earns the test. Asserting only that the item exists would pass against a host that applied the first command and dropped the second — which is exactly what the previous rule did, loudly, and what the pre-frame check did, silently.

a_frame_whose_later_mutation_fails_is_undone_whole

ADR 001 clause 4, delivered (improvise-yqpd). This entry keeps the hole it characterized, because the inversion is only legible against it.

The three assertions flipped. The removal is undone, the cell reads its pre-frame value — Some(Number(1.0)), which is neither 5.0 (the frame's own write landing) nor None (the removal surviving), and that middle value is the whole difference between a rollback and an abort — and the frame is refused out loud as Rejection::ChainBroken. A fourth assertion was added that the previous version could not have: the record counter goes back too, or the client's next batch is refused for a write that never happened.

The mechanism, and where it deviates from the ticket. The ticket asked for snapshot-and-restore at App::apply_effects. What landed is App::snapshot_document / App::restore_document — the workbook, its dirty marker and the two counted tiers, the record tier riding inside the workbook's DataStore — driven from Host::commands, which takes the snapshot before the frame and restores it on failure. It is not taken inside apply_effects itself, and the reason is cost: apply_effects runs on every keypress, and an unconditional Workbook clone there would put a full document copy on the held-key path that gotcha 24's mcq4 work exists to keep cheap. The primitive is on App, so a future RunScript opens the same transaction; only the trigger is at the frame.

What counts as a failure, and the split the ticket did not name. Two things: an effect that sets abort_effects (Rejection::MutationRefused), and a command whose record no longer resolves (Rejection::ChainBroken). The second is the interesting one. The ticket says interleaving the record check with the applies would close clause 4's silence for free but would sell the legitimate double-write — a frame writing one record twice works only because the second write is compared to the record's pre-frame counter. That is true of the counter half of the record tier and not of the structural half: writing a record twice cannot make it stop resolving. So the record tier splits — does it resolve is asked again before every mutation (reduce::broken_chain), is its counter what you claimed stays pre-frame — and clause 4 gets its loud failure without paying the price the ticket costed. a_later_write_in_a_chain_still_answers_for_its_own_record arm 3 still passes, which is the guard that the trade was not taken by accident.

Injected faults, each watched to fail. Making undo_frame restore nothing fails this test alone. Making broken_chain return None unconditionally fails this test and the mirror test.

A first version of this entry claimed broken_chain's create-exemption was "correctness-preserving but not currently observable", and that was wrong in both halves. Review built the fixture it said did not exist and found the exemption was swallowing a write — see a_frame_whose_write_would_land_nowhere_is_refused_rather_than_dropped. The exemption is gone from broken_chain and stays in the pre-frame check, where it belongs. The lesson worth keeping: "no test fails when I remove it" is evidence the tests are thin, not evidence the code is inert — the honest next step is to build the discriminating fixture, and the reasoning that excused not building one here ("resolve_record falls back to the layout, so the case cannot arise") was an argument about the common case presented as an argument about all of them.

---

The hole this replaced, kept because the inversion is only legible against it:

The contract says a frame is all-or-nothing. It was not: App::apply_effects aborts without undoing (gotcha 21 — an abort is not a rollback), so a chain whose later mutation fails leaves the earlier one standing. [RemoveItem("North"), SetCell(the North record)] is the shape: the removal lands, the write then resolves to nothing, model_effects returns an empty batch, and the frame is reported accepted having applied half of itself. Both the partial application and the silence are open.

This test did not pin that, and the way it failed is the repo's own signature defect sitting on the one test whose job is to fail loudly if anyone believes the hole is closed. It asserted that North was removed and that nothing was rejected — both true of any wholly successful frame. The independent reviewer retargeted the second command from the removed North to the live South, making the frame succeed completely, and the test still passed. It could not tell half-applied from fully-applied, so it would never have inverted when clause 4 landed; it would have gone on passing. The missing assertion is the one that names the hole: the second mutation did not land.

It is unreachable from any client here — the only producer of a multi-command frame is keys::commit, which emits one mutation among view commands — which is what makes it safe to document rather than rush.

The frame claims North at its current counter, so it passes the check and fails at apply time — which is the only way to reach the hole. A frame refused by the check never applies anything and would be testing clause 2 instead. That fixture note survives the inversion unchanged: it is still the only route to the arm.

a_rolled_back_load_leaves_no_ghost_view_and_no_false_status

Clause 4 was delivered for the document and the mirrors and NOT for the host's own session slice, found by an integration review of the first two commits. App::restore said view_state was "deliberately untouched", citing improvise-yqpd's design note: AddCategory writes its refusal into view_state.status_msg and THEN aborts, so rolling that back would undo the explanation along with the damage.

That note is about an apply_effects-level rollback, which is not what landed. At the frame level the asker receives ServerFrame::Rejected(reason) — the explanation is delivered on the wire, addressed to the client that earned it — so the host's status line has no explanatory duty for a client's frame. The note still governs the host's own key path, which aborts without undoing exactly as gotcha 21 describes, and is untouched here.

Three measured consequences of a refused frame, all now closed: the TUI host's cursor still moved and cursor_epoch was still bumped; a rolled-back LoadModel left view_state.active_view naming a view of the discarded document; and status_msg kept the rejected frame's sentence, so a frame that did nothing claimed it had loaded a file.

The fixture had to be renamed before the test could see any of it. host_over_a_loadable_document built its incoming workbook with Workbook::new, whose default view is Default — the same name the outgoing document uses — so a ghost active_view was indistinguishable from a correct one. This is gotcha 39's trap one level up, and the incoming view is now Loaded. Checked first: neither existing user of that fixture depends on the names matching, and gotcha 39's own same-name fixture is a_document_shaped_differently_under_the_same_view_name in project.rs, which is untouched.

Why restoring the whole slice is safe rather than clobbering a TUI user's in-progress typing: Host::commands is synchronous, so the snapshot and the restore bracket only the frame's own writes. There is no "in between" for a keystroke to land in.

Injected fault: dropping self.view_state = snapshot.view_state from App::restore fails this test and only this test.

a_refused_mutation_rolls_back_its_predecessor_and_the_abort_flag

The Rejection::MutationRefused arm — the one the first hand-back wrongly called unreachable — plus the batch-outcome flags going back with everything else.

batch_aborted was left true after a rolled-back frame. Inert at the time, since every Command::Model resets it at the top of apply_effects, but it is the same shape as the stale-flag bug two commits earlier and App::restore is its natural home. App's own doc comment names four more "last batch" fields and says last_reshaped_views is "the shape batch_aborted takes"; restore was clearing three of the four and neither snapshotting nor restoring any of them. All six now round-trip through AppSnapshot, which is why the type was renamed from DocumentSnapshot — it is no longer only the document.

The fixture detail that matters: a duplicate AddCategory is idempotent, so the obvious way to reach the refusal proves nothing. This fills to MAX_CATEGORIES (12) and overflows. And the predecessor is an AddItem rather than a SetCell, because adding eleven categories reshapes the view enough that a record-addressed predecessor is refused RecordGone first — hiding the arm under test behind an earlier one.

Injected fault: dropping self.batch_aborted = snapshot.batch_aborted fails this test and only this test.

a_frame_whose_write_would_land_nowhere_is_refused_rather_than_dropped

Found by reviewing improvise-yqpd's own work, and it was a hole inside the fix for the hole. The first version of reduce::broken_chain carried a create-exemption — the same will_be_created_by_the_frame predicate the pre-frame check uses — on the reasoning that the two must not drift. The hand-back recorded that arm as "correctness-preserving but unobservable", because removing the filter failed no test.

That reasoning was wrong, and the fixture that shows it is the one the hand-back said did not exist. [AddItem("West"), HideItem("West"), SetCell(West)] observed rejected=false landed=false: the frame reported accepted, with the write silently gone — precisely the partial-and-silent failure clause 4 exists to eliminate, surviving inside clause 4's own implementation.

The argument that settles it. App::resolve_cell is gated by resolve_record, so resolve_record(hash).is_none() is exactly the condition under which model_effects returns an empty vec and the write vanishes. And SetCell/ClearCell are the only variants records_named names, so there is no command for which a non-resolving record still does something useful. Refusing is therefore strictly better than dropping, everywhere.

Scope: this closes the record-addressed INSTANCE, not the class. broken_chain asks records_named, which names only SetCell/ClearCell, so a frame can still apply partially and silently through a command that names no record. Two measured as accepted: [DeleteView("Scratch"), SetNumberFormat{view:"Scratch"}], where the effect runs and no-ops inside apply; and [RemoveCategory("Region"), AddItem{category:"Region"}], where the effect fails and writes its error to the host's status line without aborting, so the client is never told. Closing the class needs effects to report failure rather than narrate it — roughly fifty of them — which is a different change from clause 4 and is stated here rather than implied.

The exemption belongs at CHECK time and nowhere else, and that asymmetry is the point: before the frame runs, a record the frame will create legitimately does not exist, so [AddItem(East), SetCell(East)] must pass — kb.pl's the_create_exemption_is_a_property_not_a_list is unchanged and still describes stale_records_in_a_frame. At apply time the record has either been created or it has not, and "has not" means the write is lost.

Injected fault: restoring any exemption to broken_chain fails this test, plus a_frame_whose_later_mutation_fails_is_undone_whole and the mirror test.

a_frame_of_view_commands_is_not_refused_by_an_earlier_batchs_abort

The second review finding, and a live bug improvise-yqpd introduced. App::batch_aborted is reset only inside apply_effects (gotcha 21), and Host::commands read last_batch_aborted() after every command in a frame. A command that runs no effects — every view command, and Command::SwitchView, which apply_checked routes to apply_switch rather than through apply_command — leaves the flag exactly as some earlier batch left it.

So a TUI :w /bad/path raised the flag, and the next frame a connected browser sent was refused MutationRefused and rolled back, having done nothing wrong. Reproduced by aborting a pathless effect::Save directly on the App and then sending [SetSelected].

Two arms, because frame-level gating alone is not the fix. The first arm is a pure view frame, which the Command::Model-presence gate in commands handles. The second is the mixed frame [SetSelected, SetCell], which that gate lets through and which only the per-command mutation && guard answers — the view command runs no effects, so the flag under it is still the stale one. Injecting if { let _ = mutation; self.app.last_batch_aborted() } fails this test and only this test, which is what shows the per-command guard is load-bearing rather than belt-and-braces.

The same Command::Model-presence predicate is why a frame of pure cursor keys now pays for neither the Workbook snapshot nor the Sessions clone — the browser's common path, one frame per keypress.

the_mirror_settles_its_mode_against_every_grid_it_is_later_projected

improvise-el9h. ClientSession::fold settled view.mode against self.grid — the grid that session was last projected — and project_diff then assigned self.grid = desired and never settled again. The client does not have that shape: ClientState::fold calls settle_mode() on every fold including Command::Project, so when the new view's grid arrives the client re-settles and the mirror does not.

[SwitchView(records-shaped view), ChangeMode(Normal)] is the shape — precisely the case PR #98's settle exists to serve. Demonstrated before the fix: the client ended on RecordsNormal and the mirror on Normal. The fix is settle_mode called from fold and at the end of project_diff once the grid has moved, which also closes the three bypass doors (apply_switch, adopt_deleted_views, adopt_cursor_epoch write the mirror with bare reduce_view): whatever they leave, the next projection settles.

Two preconditions, and one of them caught a broken fixture on its first run. The test asserts the client starts on Normal, so the settle has somewhere to move the mode from. And app_with_a_records_shaped_second_view asserts the second view really reaches records mode — which failed immediately, because the view was built with View::new and a fresh View carries no axis entry for _Index/_Dim, so is_records_mode can never fire on one. Without that assertion the test would have compared two modes that were both Normal for a reason having nothing to do with the bug, and passed against the unfixed code.

The yardstick is a real improvise_protocol::ClientState fed the same projections in arrival order, not a hand-written expectation: the property is that the mirror and the client run the same function over the same stream.

a_failed_frame_puts_the_askers_mirror_back_on_the_view_it_was_on

Clause 4's least obvious cost, in the ticket's own words: rolling back the App is not enough. Host::command projects per command and apply_switch / adopt_deleted_views / adopt_cursor_epoch move mirrors inline, so by the time a later command fails, every session's mirror has already moved. Host::commands therefore snapshots the whole Sessions beside the document and restores both.

The first version of this test was vacuous, and the injection is what showed it. It seated a bystander on a second view and asserted the bystander was told nothing and its mirror was unmoved — both of which hold when undo_frame restores nothing at all, because the frame's projections are discarded from out either way and nothing ever moved that bystander. It answered did the frame return early where it meant was the mirror put back.

The rewrite aims at the one mirror move a failed frame really makes and that nothing in the stream can ever correct: the asker's own SwitchView. An accepted switch is relayed to nobody (gotcha 36), so a mirror left on the switched-to view diverges from a client that was told the frame was refused, permanently and silently. The fixture asserts up front that the mirror starts on Default — the direction the default cannot supply — so the closing assertion is not satisfied by a mirror nothing ever wrote. Removing self.sessions = mirrors while keeping the document restore fails this test and only this test.

a_frame_that_creates_the_record_it_writes_is_not_a_broken_chain

The other direction: clause 4 must not fire on [AddItem("West"), SetCell(West)], which is the exemption ADR 001 states as a property — a record that neither resolves nor is claimed is one the frame is creating.

Be clear about what this does and does not catch. It catches broken_chain being asked against the pre-frame document instead of the running one, which is the plausible implementation error (passing the snapshot where the live App belongs) and would refuse every data-dependent chain. It does not catch the exemption filter being dropped, for the reason recorded above: resolve_record falls back to the layout, so West already resolves once AddItem has run. A fixture that discriminated the filter would need a created record that never enters the layout; none exists in this workspace and one was not invented for it.

Helper — frame_basis

Not a test — the frame-level claim that test and the 955n tests state.

The basis a caught-up client would state for a frame naming regions: the two document tiers plus each named record's current counter.

reduce::caught_up cannot stand in for this, and the difference is exactly what a frame is: it builds the claim for one command, so a frame's records beyond the first are simply absent from it and come back RecordUnstated. A real client calls BasisCache::claim once with every record its frame names, which is what this reproduces.

Name only records that exist. claim omits a record the cache has never heard of; this helper always includes one, so pointed at a record that is not there it fabricates (hash, 0) — a claim no real client could compose, and precisely the input that flips the create exemption in CheckedBatch. For a record the frame is creating, leave it out, which is what a client would do.

an_accepted_switch_is_relayed_to_nobody

The successor to a_bystander_is_sent_the_broadcast_and_the_asker_is_not, which pinned the pre-improvise-jytm relay (Copilot, PR #10). An accepted switch is now owed to nobody: the asker folded it, and no other client's view moves.

It asserts the bystander gets no frame at all, not merely no SwitchView. A relay that vanished while the projection fan-out still repainted every client would pass the narrower check and still be pushing one client's view change at the rest.

a_geometry_command_relays_the_cursor_reset_to_the_asker_too

The other audience, and the reason Broadcast carries one rather than the relay deciding. A geometry change invalidates every cursor into the document (improvise-gvnb), and the asker's is not exempt: a Command::Model is the one kind a client cannot fold optimistically, so nothing has reset it locally and skipping it would leave the very client that asked positioned in a grid it no longer has.

a_stale_batch_is_refused_to_the_asker_alone

A refused batch is answered to the asker and nobody else (ADR 001). Nothing happened, so there is nothing to tell the others — and the asker is told only which class of staleness it was, because it already holds what is current.

folding_the_relay_puts_a_refused_asker_back

What Broadcast::To is for, asserted end to end through Host rather than through Sessions: a client that folded a view name the document refused is put back by the frame the host sends it, so the cursor its next grid is positioned by is read under a key the document has (gotcha 24).

The client slice is folded before the command is handed to the host, which is what makes the assertion mean anything: a test that never folded would see "Default" whatever the relay did. The bystander is asserted silent in the same test, because a ToAll correction would also put the asker back and would be wrong.

a_document_swap_sends_every_client_a_fresh_snapshot

AC of gotcha 39: a command that replaced the document owes every client a fresh Snapshot, not a diff — including the client that did not ask.

Helper — host_over_a_loadable_document

Not a test — the host that test and a_load_that_fails_is_not_a_rebase drive.

A host whose document can be replaced out from under its clients: a second document sits in the storage backend under a name LoadModel can reach.

a_client_can_ask_its_host_to_save

improvise-368h. The bug: effect::Save is EffectKind::ViewOnly — correctly, since it writes this host's path — so the derivation that builds ModelCommand out of the EffectKind::Model effects could not reach it (gotcha 38). There was no save on the wire at all, so no client on any transport could cause its host to write: a websocket session edited the document in memory and the file on disk never moved unless a human was at a TUI that was not there.

The derivation was not wrong; it was complete over the mutations and saving is not one. So the variant is added rather than derived, and it carries no path: a client cannot see the host's storage, and which document a host is hosting is the host's own fact.

a_save_with_nowhere_to_write_is_refused_and_told_only_to_the_asker

improvise-368h: the "what if there is no path" half. Not a panic and not a silent no-op — the two answers the ticket ruled out — but the vocabulary that already exists for "nothing happened and here is why", addressed to the asker alone (gotcha 39).

The second client is not decoration: a refusal that leaked to the bystanders would be telling them about a failure of somebody else's host-local request, and this asserts it does not.

a_write_that_fails_is_refused_rather_than_reported_as_saved

improvise-368h: a write that was attempted and failed must not be reported as a success. Save signals failure by aborting its batch (gotcha 21), and the batch here is exactly [Save], so an abort is that failure and nothing else.

The fixture fails for a reason that is real rather than mocked: a PhysicalFs pointed at a directory that does not exist. It creates nothing, so the test leaves no trace on disk.

every_frame_survives_a_json_round_trip

The wire is JSON to start with (the ticket's choice, kept), so both directions must survive a round trip through it — including the one point at which ViewState is serialized (gotcha 25).

The inbound half carries a payload-bearing variant on each side of the envelope (doctrine(populate_before_roundtrip)): a Viewport and a Command carrying a CellValue::Text, which is the arm the outbound half cannot reach — a text cell aggregates to nothing under a None-axis category, so it never becomes a CellDisplay.

Note: host.rs also carries a second a_load_that_fails_is_not_a_rebase (doctrine(check_the_checker): inject the fault the test targets — a load that does not happen — and the snapshot must not appear; a test that saw Snapshot on every command would pass a_document_swap_sends_every_client_a_fresh_snapshot). It shares its name with the ui/project.rs test of the same name, which is why there is one heading for both above.

---

ui/cat_tree.rs

the_tree_carries_the_virtual_categories_and_measure_includes_formula_targets

improvise-vjms: the panel lists the virtual categories alongside the regular ones, and _Measure's items are derived — explicit items ∪ formula targets (gotcha 7). An implementation reading category("_Measure").items passes every other assertion in the module and silently drops Profit, which is the failure improvise-ete was closed for proposing.

---

command/keymap.rs

f1_and_question_mark_both_open_help

improvise-th7m: F1 was bound twice on consecutive lines — first to a bare enter-mode with no args, then to enter-mode ["help"]. The first was dead, because Keymap::bind inserts and the later binding wins; had it ever been the surviving one it would have failed require_args at dispatch and made F1 do nothing.

The risk in deleting a line of a duplicated pair is deleting the wrong one, so this pins which binding F1 is meant to carry: the same one ? carries, which is what identified it as the intended survivor.

every_bound_command_name_is_registered

improvise-9cn: Keymap::dispatch resolves a binding through registry.interactive(name, args, ctx).ok()?, so a binding is a silent no-op key in two ways, not one: the name may not be registered, or the args may not parse. Both walks live in this test.

The args half matters because improvise-s13 moved bindings out of the nullary class the name check covers: enter-mode ["export-prompt"] and enter-mode ["formula-edit"] carry a mode name that parse_mode_name must accept, and a typo like "export_prompt" compiles, registers, and produces a dead Ctrl-e. (Gotcha 15.)

It builds the context in the mode whose keymap it is walking, because a few parsers read ctx.mode — commit-buffer item resolves its target category off AppMode::ItemAdd.

improvise-piyp. This entry is about the test's reach, not its logic — the logic was right and demonstrably so, which is what made the gap hard to see.

It had two blind spots, both found by injection rather than by reading:

• the walk matched Some(Binding::Prefix(_)) | None => {}, so it never descended into a prefix sub-keymap. Everything under g, y and Z was unchecked, in every mode — and RecordsNormal reaches Normal's three through Keymaps::prefix_keymap, so it was unchecked twice over.
• its key space was a second copy of tests_support::probe_keys_pub, identical but for the two F-keys the public one carries. So a name bound on any function key was invisible here while dump_every_binding probed it.

Injected at 523d571: one unregistered name under the g prefix and one on F(5) in Normal. cargo test --workspace came back 1595 passed, 0 failed — byte-identical to the clean baseline. The positive control that separates "untested path" from "broken test" is the same bogus name on Char('~'), a probed non-prefix key: that fails. The guard worked; it could not see there.

Both are closed here. The walk is reachable_names, which recurses into Binding::Prefix — terminating because prefix_keymaps_are_flat says the nesting is one deep — and probe_keys now delegates to probe_keys_pub rather than restating it, which is the part that matters: two lists of "the keys a test probes" is the same disease as two keymap spines, and the F-key gap was exactly the drift between them. probe_keys_pub covers F(1)..=F(12) rather than the two that happen to be bound, since the point is to see a binding nobody expected.

Re-running the two injections against the fixed test: the prefix one fails with Normal binds "no-such-command-under-a-prefix", which no registry entry provides, and the F-key one with the matching message for F(5).

Worth noting for contrast, because reading either test alone misleads: nothing_declined_is_also_composed in resolve.rs already recursed into prefixes and into parents. The resolution side was guarded where the registration side was not, and only comparing the two shows which half is real.

prefix_keymaps_are_flat

improvise-jc8: an armed prefix is stored as the key that opened it and resolved against the CURRENT MODE's keymap. That is only correct while prefixes are flat — a Prefix binding nested inside a sub-keymap would be resolved against the mode map, where it does not exist, and the second half of the sequence would silently do nothing. Adding a nested prefix must fail here rather than in the terminal.

every_panel_keymap_shares_the_same_spine

improvise-4pg: the three panel keymaps were written out separately, so a spine binding could be changed in one and forgotten in the others. They share PanelKeymap now; this pins what "the spine" is, including that each panel's cursor keys move its own cursor. (Gotcha 12.)

every_mode_with_panel_toggles_binds_all_of_them

Every panel's toggle key is bound in Normal and in all three panel modes, so focus can move between any two and the focused one closes. A fourth panel gets all four keymaps from one Panel::ALL entry.

panel_toggle_keys_dispatch_to_real_effects

Panel::name() has to spell what the registry's panel parser accepts. A lookup-only assertion would not catch a mismatch — dispatch swallows an unresolvable command with .ok()? — so the toggle key must come back with real effects.

every_text_entry_mode_types_into_the_buffer_its_prompt_names

improvise-k8h: the buffer name used to be written on both sides of the crate — AppMode's MinibufferConfig names the buffer the prompt describes, and the keymap names the buffer append-char edits. If those disagree the mode prompts over one buffer while every keystroke edits another, with nothing to notice. TextEntry now reads the name off the mode, and this pins the result for every text-entry mode. (Gotcha 11.)

Helper — text_entry_modes_with_exits

Not a test — the list that test and the two below it walk.

The seven modes that carry a MinibufferConfig, each with the keys that commit or leave it. SearchMode is not among them: it is Normal plus a flag, so it has no AppMode variant and no minibuffer config to agree with.

every_typing_keymap_belongs_to_a_text_entry_mode

The list in text_entry_modes_with_exits must stay complete: every keymap that types into a buffer belongs to a mode whose minibuffer names that same buffer. Only SearchMode, which has no AppMode variant, is exempt.

text_entry_modes_clear_the_buffer_they_type_into

Every minibuffer mode types into exactly one buffer, and every key that commits or leaves must clear that same buffer on the way — a buffer left behind pre-fills the next entry with the last one. SearchMode is excluded on purpose: it has no commit and its query is owned by exit-search-mode, not by clear-buffer.

The buffer is derived from each mode rather than restated in the test — every_text_entry_mode_types_into_the_buffer_its_prompt_names owns pinning what it should be (improvise-k8h).

minibuffer_enter_dispatches_to_a_registered_commit

improvise-6t3: the three commit commands collapsed into a single commit-buffer <kind>. dispatch swallows an unresolvable command name with .ok()?, so a lookup-only assertion would still pass had the rename missed the registry — Enter has to come back with real effects.

dump_every_binding

The instrument doctrine(binding_dump_diff) asks for: every (mode, key, modifier) lookup across all the modes, written to the file named by IMPROVISE_BINDING_DUMP.

Most bindings have no direct test, so a green suite is not evidence that a keymap refactor preserved them. Dump before, dump after, diff. It is inert without the variable: a test that writes a file nobody asked for is a surprise, not a check.

This is what proved the keymap move to improvise-protocol changed nothing — 4021 resolved lookups, byte-identical either side — and the diff was shown to be discriminating by renaming one binding and watching eight lines move.

---

command/cmd/commit.rs

Not an assertion — doctrine(binding_dump_diff)'s instrument, and the evidence improvise-636d actually rests on, because a green suite is not evidence for a keymap refactor.

Diffed across the global change: 84 added lookups, 0 removed, 0 changed. No binding anywhere answers differently than it did, which is the property that matters for a change that moves bindings between tables. The additions are 16 rows of ? reaching the three panels and TileSelect, 1 of Help's new ?, 24 of the panel toggles reaching Help and TileSelect, and 40 that are not a binding change at all — the widened probe_keys_pub hitting ImportWizard's KeyPattern::Any on F(3)..=F(12).

: produced zero rows, which is the result worth keeping: every mode that now inherits it already bound it locally, so moving it into the global changed no answer anywhere. That is what "the duplication was real and consistent" looks like from the outside — and it is also why nothing in the suite could have caught the two spines drifting apart, had they ever done so.

invalid_formula_reports_the_parse_error_not_success

improvise-6t3: the commit pushed set_status("Formula added") after AddFormula, so the parse error that AddFormula::apply writes to the status bar was immediately overwritten. An unparseable formula was dropped while the UI reported success.

The add effect is the only party that knows whether it worked, so it has to get the last word on the status line.

commit_and_advance_threads_edit_mode_to_enter_edit_at_cursor

CommitAndAdvance must thread its edit_mode through to the trailing EnterEditAtCursor effect so the post-commit re-edit lands in the mode the keymap requested. The command never reads ctx.mode.

commit_save_writes_the_model_to_the_typed_path

improvise-lae: the save prompt has to commit to SaveAs and not to ExportCsv, which is what makes it a separate mode from ExportPrompt rather than a second binding onto it — ExportCsv would write CSV under whatever .improv name the user typed.

SaveAs and not Save, because Save writes the path the document already has, and the whole reason this prompt exists is that there isn't one.

commit_save_on_an_empty_path_writes_nothing

An empty prompt must not reach SaveAs: SaveAs(PathBuf::from("")) asks the backend to write to "", and the user then reads whatever I/O error that produces instead of "you typed nothing".

commit_export_bails_on_an_empty_path

improvise-z05r's first arm. CommitExport did not bail on an empty path where CommitBuffer's three kinds all do (gotcha 20: buffer to trim to bail-on-empty to dirty to status is the shared spine), so Enter on an empty export prompt asked the backend to write to "" and the user read whatever I/O error that produced.

The same change reorders the batch: the mode change now comes first, because ExportCsv aborts on a failed write as of z05r's other arm, and an abort at the tail would strand the user in ExportPrompt — where draw_bottom_bar renders the minibuffer instead of the status line, so the export error is never drawn at all and the operation appears to do nothing (gotcha 21). That ordering is asserted in commit_export_produces_export_and_normal_mode, by index and not by presence.

---

command/cmd/fit.rs

the_command_installs_a_formula_the_document_evaluates

The end-to-end claim of improvise-vx3, asserted against a real document: a command fits, the formula lands through the ordinary add_formula path, and the new measure evaluates to numbers.

Helper — workbook

Not a test — the document every fit.rs test fits against.

A document a tree can learn exactly — Score steps at Cost 50 — in the units a real document uses.

Every category is a Label, so the only feature is the numeric measure Cost and the champion cannot reach an unresolvable one. An earlier version left Region regular with one item and claimed simplify would prune the constant one-hot; it does not, because build_categorical records (0.0, 1.0) for every one-hot whatever the column holds — so these tests were passing on seed luck (Copilot on PR #25). See improvise_gp::fit's own fixture for the same correction.

the_status_line_reports_quality_against_the_baseline

improvise-vx3 is emphatic that the rule alone is not the deliverable: a tidy tree that generalizes no better than the mean is worse than useless. So the status line carries the held-out comparison, what the rule leans on, and the seed that reproduces it.

a_refusal_names_what_went_wrong_and_stores_nothing

A refusal reaches the user whole. "fit failed" is not actionable; naming the measure that does not exist is (improvise-8xhy's argument, arriving at the place a user meets it).

the_seed_argument_changes_the_search

Gotcha 37b: the seed is half of what reproduces a fit, so it is an argument — and a different one really does search differently.

It asserts on the rule, never on the batch's debug string. The first version compared effects_debug over the whole vec, which was vacuous: status_line prints seed {:#x}, so the two strings differ because the seed was printed, and fit_measure could have ignored request.seed entirely with the test still green (doctrine(check_the_checker)).

AddFormula has exactly two fields — the formula text and the target category — and the seed reaches neither, so its Debug cannot differ for the reason the batch's did. (A substring guard was tried and is wrong: the seeds 1 and 99 occur inside perfectly ordinary thresholds.)

the_seed_the_status_line_prints_is_a_seed_this_command_accepts

Copilot on PR #25. The status line advertises the seed as {:#x} and tells the user that is what reproduces the fit, so the one instruction it gives has to work on the value it just showed them — a decimal-only parser made copying it back an error.

Asserted as a round trip through the status line, not against a hand-written "0x…": a literal would still pass if status_line changed how it spells the number, which is the half that breaks.

---

command/cmd/grid.rs

drill_into_formula_cell_returns_data_records

Drilling into a formula cell (Profit = Revenue - Cost) must return the underlying data records, not an empty result set. The formula target coordinate is stripped from the drill key so that matching_cells finds the raw data backing the formula (gotcha 10).

drilling_fixes_its_coordinates_on_filter_not_page

improvise-dz8: a drilled coordinate is a constraint of the drill, not a dimension the user may page through, so it lands on Filter. It used to land on Page, which put it in the tile bar and under [/] as if the user could move it (gotcha 27).

---

command/cmd/mode.rs

enter_mode_export_prompt_sets_mode_and_clears_its_buffer

The enter-export-prompt shim is gone (improvise-s13); the generic enter-mode carries the mode name instead, and clears the prompt's buffer on the way in.

enter_mode_clears_a_search_keyed_minibuffer_through_write_buffer

improvise-gdl made write_buffer the single accessor for a runtime-named buffer, because "search" is not an entry in the buffers map but its own ViewState field. EnterMode built a SetBuffer directly, so a mode whose buffer_key is "search" would clear a map entry nobody reads and leave the previous query on screen — the exact desync gdl closed. No shipped AppMode names that buffer yet, so the mode here is constructed by hand; that is what makes the test a demonstration rather than a restatement.

edit_or_drill_passes_records_editing_mode_through

EditOrDrill must trust its edit_mode parameter rather than checking ctx.mode — the records-normal keymap supplies records-editing, but the command itself never inspects the runtime mode. This is the parallel of the deleted enter_edit_mode_produces_editing_mode test for the records branch.

enter_edit_at_cursor_cmd_passes_target_mode_to_effect

EnterEditAtCursorCmd must hand its target_mode straight through to the EnterEditAtCursor effect — the keymap (records o sequence or commit-and-advance) decides; the command never inspects ctx. It fronts the pair with RebuildLayout so the cursor is read against a layout that includes whatever ran earlier in the sequence (improvise-ehz, gotcha 23).

---

command/cmd/navigation.rs

page_cycling_skips_filter_coordinates

improvise-dz8: [ and ] page through Page-axis categories. A coordinate the drill fixed sits on Filter and must be invisible to them, or the user could page out of the drill they are inside.

enter_advance_at_bottom_right_emits_abort_chain

At bottom-right EnterAdvance has no place to go, so it emits a single AbortChain effect. Trailing effects in a CommitAndAdvance batch (EnterEditAtCursor) are then skipped, which is how "Enter at bottom-right commits and exits editing" is realised.

---

command/cmd/panel.rs

cycle_panel_focus_advances_from_formula_to_category

Bug improvise-dqn: CyclePanelFocus always picked the first open panel, ignoring the currently focused panel. With formula + category open and focus on the formula panel, cycling must move to the category panel.

create_and_switch_view_names_incrementally

improvise-aaw asked for this command to be replaced by a keymap Binding::Sequence [create-view X, switch-view X, enter-mode normal]. It cannot be: a Sequence carries literal args, and the name here is computed from ctx — so a Sequence would hard-code one name and the second n in the view panel would collide with the first.

The test was called ..._names_incrementally and asserted only that CreateView/SwitchView/Normal appeared — it never looked at the name, which is the one thing that makes this command a command and not a composition. Now it does, at two different view counts, because a single count cannot tell "derived from the workbook" from "constant".

---

command/cmd/registry.rs

every_entry_constructs_a_command_with_its_registered_name

improvise-9cn / improvise-61f: every registration derives its name from a prototype instance, so a typo in the name is impossible. What the types do not catch is the commands whose name() branches on a field — ViewNavigate, TileAxisOp, ToggleGroupAtCursor, CommitAndAdvance. Their prototype and the value their constructor builds are the same type, so registering ViewNavigate { forward: true } while building forward: false compiles cleanly and files the command under the wrong name, where dispatch silently misses. Constructing every entry both ways and comparing names is the only check that reaches that class. (Gotcha 15.)

Some entries are deliberately one-path — commit-and-advance-right reads the edit buffer and cursor, so its parse fn always errors — but every entry must be reachable by at least one path, or its name goes unchecked.

registered_names_are_unique

A variant mix-up does not show up as a name mismatch — it shows up as a collision. Registering ViewNavigate { forward: false } where forward: true belongs files a second entry under "view-back" and removes "view-forward" from the registry entirely; parse takes the first match, so every surviving entry still agrees with its own name. Uniqueness is what catches it.

the_context_reports_each_panels_own_open_flag

improvise-y4xg and improvise-qsmp, two shapes of one defect. registry.rs mapped Panel to a CmdContext field in two places, and App::cmd_context copies the three flags out of ViewState in a third. Both were demonstrated unguarded: swapping Formula and Category at the two registry sites left the workspace at 1400 passing, and hardcoding the category_panel_open propagator to true left it at 1394.

Nothing exercised either, because cmd/mod.rs::test_helpers sets the panel flags explicitly on the context it builds — the command tests construct the value they then assert on, so no test ever drove the real propagator.

The registry mapping is gone: both sites call CmdContext::panel_open(panel), the one mapping on this side of the wall. That leaves two permutable sites, and the test is aimed at both from a path neither controls — it writes view_state's three fields by name and reads back through the enum accessor on a context built by the real App::cmd_context. PR #70's review caught the weaker shape of this: a panel test that zeroed and read through the same four accessors passed a consistent permutation.

Injected fault: swap Panel::Formula and Panel::Category in panel_open. Fails. Injected fault: hardcode the category_panel_open propagator to true. Fails.

toggle_panel_visibility_reads_the_panel_it_names

The behavioural half, and the one that reaches the registry. Its interactive arm passes currently_open straight through, so with exactly one panel open the command built for each of the three names carries its own panel's flag. Both injections above fail here too.

a_focused_open_panel_closes_and_a_closed_one_opens

improvise-qsmp's own scenario: toggle-panel-and-focus decides close only when the panel is both open and focused, so the case that separates a right propagator from a stuck-true one needs the mode set to the panel's own. With the flag stuck true, pressing a panel's key on a closed panel decides to close it and the panel becomes unopenable by its own key — which is what this test's second arm names.

Helper — open_only

Not a test. It spells formula_panel_open, category_panel_open and view_panel_open as fields rather than looping over Panel::ALL through an accessor, and that is the whole evidentiary value: a permutation in any mapping between the Panel enum and those three booleans has to disagree with this helper.

---

command/cmd/text_buffer.rs

writing_the_search_buffer_targets_the_search_query

Writing the "search" buffer means writing the search query, and write_buffer is the one place that knows it (improvise-gdl). Every buffer-writing command routes through it, so clear-buffer search clears the query rather than parking an unread entry in the map.

colon_wq_saves_then_quits

improvise-git proposed deleting the SaveAndQuit command and binding ZZ to a Binding::Sequence [w, force-quit] instead — the repo's "do not name a command AAndB" principle.

That premise is incomplete, and this test is why. wq is not only a keybinding: it is a registered command name, so :wq reaches it through the registry, and Quit's own unsaved-changes message advertises it ("Use :q! to force quit or :wq to save+quit"). A keymap Sequence is not reachable from the : line, so deleting the command deletes the ex-command with it.

Keeping the command and adding the Sequence would leave two independently-written spellings of "save then quit", which is the duplication this codebase keeps curing rather than creating. So this pins the surface, and the conversion is declined on it.

colon_enter_edit_at_cursor_is_not_snapped_back_to_normal

improvise-6sq: ExecuteCommand appends change_mode(Normal) unless some effect reports changes_mode(). EnterEditAtCursor sets view_state.mode but did not override it, so :enter-edit-at-cursor editing produced [RebuildLayout, EnterEditAtCursor(Editing), ChangeMode(Normal)] — the trailing snap-back undid the edit mode and the user never got a cursor in the minibuffer.

Asserting on changes_mode() alone is not enough here: the appended ChangeMode(Normal) reports true itself, so the assertion has to name the snap-back it is ruling out.

colon_import_stays_in_the_wizard_on_success

improvise-toc: StartImportWizard swaps the mode only when the file parses, so a per-type changes_mode() cannot describe it. Reporting false made ExecuteCommand append a return-to-Normal on top of the success path: :import d.json opened the wizard and was immediately snapped back, leaving the wizard drawn under the Normal keymap. (Gotcha 31.)

colon_import_falls_back_to_normal_on_failure

The other half of the same bug: reporting true unconditionally would strand the user in CommandMode when the import fails. A failed :import must still fall back to Normal.

colon_view_back_respects_whether_history_exists

The ViewBack half of improvise-toc: it restores the stored frame's mode only when the back-stack is non-empty. With history present, :view-back must keep the mode it restored; with an empty stack it falls through to Normal like any other no-op command.

---

command/cmd/wizard.rs

name_model_enter_builds_the_model

Enter on the name step builds the workbook into the wizard rather than advancing the step machine; every other printable key edits the name (gotcha 22).

done_confirms_on_enter_and_steps_back_on_esc

On the confirmation step Enter installs the built workbook and Esc steps back — it does not throw the import away, and no key closes the wizard outright (improvise-907, from improvise-c6j).

a_formula_owned_cell_is_projected_as_derived_and_its_siblings_are_not

improvise-ir7l. project::cell_id decided Record vs Aggregate purely on whether a regular category sat on Axis::None; it never asked whether the _Measure coordinate named a formula target. So a formula cell was a Record, render.rs::editable said true, and a write to it was accepted, stored and then shadowed forever by formula_cache.

The fault injected: replacing the derived lookup with None, which is the pre-fix code exactly. The test then reports Record(RecordCell { measure: Some("Profit") }) where it wants Derived { target: "Profit" }. The siblings are asserted in the same test so a producer that answered Derived for every cell could not pass.

an_aggregating_formula_cell_is_still_projected_as_an_aggregate

The negative arm of the same change, and the one a careless fix breaks: gotcha 10 says drilling into a formula cell is supported, so Aggregate must still win where the cell aggregates. Proved discriminating by returning derived before the aggregate branch — the cell then reads Derived and becomes undrillable, with nothing else in the suite noticing.

edit_or_drill_refuses_a_formula_owned_cell

The TUI half. EditOrDrill never asked the question either, so the terminal offered the same editor over the same shadowed cell. Landing only the projection half would have made the two front ends disagree, which is the defect design-principles §5 is about. Fault injected by deleting the guard: the command returns [SetBuffer{edit}, ChangeMode(Editing)] again.

a_save_clears_the_dirty_marker_on_a_client_although_nothing_is_dirty

improvise-z754, and the arm the whole design turns on. effect::Save is EffectKind::ViewOnly, so Dirty::of reports NOTHING — the test asserts that first, so it cannot quietly stop being the interesting case if Save is ever reclassified. A title bar gated on Dirty would therefore leave every client showing [+] for a document that has just been saved, forever, with nothing in the stream to correct it.

Fault injected: wrapping document_update in if !dirty.is_empty(), which is the obvious implementation and the wrong one. The test fails on the cleared arm. a_title_bar_that_did_not_move_is_not_projected is its counterweight — without it, "project the title bar every batch" would pass this one.

the_title_bar_names_the_document_without_naming_the_path

The fixture's path is /home/someone/private/books.improv on purpose: a test whose fixture is a bare file name cannot tell file_name() from the whole path, and the failure mode is a socket server handing a remote client the host's directory layout.

a_creating_column_is_named_rather_than_left_as_no_cell

improvise-poia. Both + and +cat projected None, which the protocol defines as "no projected cell" — the same conflation gotcha 42 names for aggregates, arriving a third time.

The test walks every column of a records grid and counts, rather than asserting the two creating columns alone. That matters in the direction a weaker test fails: a producer that answered Create for everything, or that made the Region coordinate column nameable, would pass a two-position check and fails this one. The fault injected — returning None for both creating columns, which is the pre-fix code — reports three nameless columns where one is allowed.

a_records_coordinate_is_text_however_it_is_spelled

improvise-tom1. project::cell_display's records arm called records_display — which returns a String — and then inferred numeric-ness with text.parse::<f64>(). A records coordinate column holds an item name, so a Year category with items 2024/2025 projected a coordinate the browser right-aligns as a number.

Nothing in the suite could see it: every records fixture in the tree names its categories Region/East/North, so the guess happened to be right. The fixture here is the whole test — a category whose items are spelled like numbers — and the measure column beside it is asserted numeric in the same test, so a producer that answered false for everything would not pass.

Fault injected: the parse::<f64>() guess restored verbatim. It reports the coordinate as numeric.

a_staged_drill_edit_reaches_its_own_session_and_no_projection

The user's ruling, pinned in both directions (improvise-tom1). A remote client must NOT see another client's staged drill edits, so grid_of must project the committed value; the session that staged one must still see its own typing, through App::display_value_of.

Asserting only the second half would pass against a producer that leaked, and asserting only the first would pass against one where staging did nothing at all — so both are here. The fault injected is the leak: teaching cell_display's records arm to prefer drill_state.pending_edits, which is the plausible "fix" this ticket exists to forbid. The projected cell then reads 999 instead of 1.

The fixture takes its DrillState::records from the layout the app really built, because an empty frozen snapshot silently empties the drill grid and the test then asserts nothing about a cell that does not exist — the first version did exactly that and panicked on expect.

a_page_axis_carries_its_items_and_a_filter_axis_does_not

The producer half of improvise-p1ym, added during self-review because a_filter_axis_is_projected_but_not_cycled builds its own PageAxis by hand — a test that fabricates the very data the code under test is supposed to produce, which is this repo's characteristic defect. Only this one can say what grid_of actually emits.

The fixture puts Quarter on Axis::Page and Year on Axis::Filter, which is the shape a drill makes (gotcha 27), and asserts the whole projected list in one comparison so a producer that emitted items for everything or for nothing both fail. Fault injected: let navigable = true, which is filtering_cats() leaking into the protocol — the exact trap the ticket names. Year comes back carrying ["2024"].

a_records_grid_has_no_column_for_a_formula_target

A records grid has no column for a formula target that is only a formula target, because GridLayout::records_columns builds its measure columns from Category::ordered_item_names — the explicit _Measure items — while add_formula adds none (gotcha 7). Fault injected: swapping ordered_item_names for measure_item_names in records_columns, which is a plausible tidy-up — a Profit column appears.

This test's original premise was FALSE, and the review caught it. It was written claiming the records branch of cell_id was safe because "a formula target lives only in measure_item_names". An item can be both an explicit _Measure item and a formula target, and improvise-poia — in the same PR — made the + column that builds exactly that reachable. So this test asserted the accident in the one configuration where the accident holds, and its own failure message named a repair for a case it could not reach. The real guards are a_records_column_a_formula_owns_is_derived_not_a_record and typing_into_a_records_formula_column_is_refused_on_the_key_path; this one now claims only what it can see.

a_records_column_a_formula_owns_is_derived_not_a_record

The hole improvise-ir7l's first fix left, found by adversarial review. A measure can be both an explicit _Measure item and a formula target — the records + column creates precisely that — and cell_id's records branch asked formula_for about nothing, so it projected CellId::Record{measure:"Profit"}. A write there is accepted, stored, and then shadowed forever by formula_cache, which is the defect the ticket exists to close, surviving in records mode in both front ends since editable is Record-only.

The repair asks the document about the record's real coordinate — GridLayout::record_cell_key, the one mapping from a records position to a model cell — never the synthetic (_Index, _Dim) key, which names no measure and so can never match a formula. Fault injected: deleting the lookup, which is the pre-review code. Profit reads back as a Record. The Revenue column is asserted still writable in the same test, so a producer answering Derived for everything could not pass.

typing_into_a_records_formula_column_is_refused_on_the_key_path

The same defect through App::handle_key and no test API at all: R, walk to +, type Profit, Enter, Esc, then i on the column just created. It exists because the projection half and the EditOrDrill half are separately breakable, and a unit test of either would have missed the other — proved by injecting each alone. Restoring cell_id while leaving CmdContext:: owning_formula reading the raw key fails on the mode assertion; the reverse fails on the identity assertion.

One fixture note that cost a debugging round: CommitAndAdvance re-enters editing on the cell it advances to, so the mode after Enter is RecordsEditing and an i there is a literal character. Without the Esc the test passes its mode assertion for the wrong reason — nothing happened at all.

a_save_tells_the_client_the_document_is_clean

improvise-z754's own stated failure mode, shipped: "every client showing a [+] for a document that has been saved". Host::save applied effect::Save and returned only a possible Rejected, never projecting, so the dirty marker cleared outside any projection and no client was ever told.

The z754 test that was supposed to cover this called sessions.project_all by hand — which is exactly the call the production path omits. It is not a test that compares a thing with itself; it is a test that exercises a call the deployment never makes, which is the subtler shape. This one goes through Host::handle(ClientFrame::Save).

Two assertions guard the fixture: the edit must first report dirty: true, and app.model_state.dirty must really be false afterwards — otherwise a host that saved nothing would pass by projecting nothing. Fault injected: returning an empty vec from the success arm. No Document update is emitted.

the_automatic_persist_tells_the_client_the_document_is_clean

The worker's half of the same defect, and a different cause: on_frame built out via frames_for and then called persist(), so the projections were built while dirty was still true. The browser is the deployment where this bites hardest, because nothing there ever sends ClientFrame::Save — the worker persists on its own behalf (gotcha 45), so this was the only path.

Fault injected: dropping the re-projection after persist(). The client's DocumentCache stays dirty. MainThread grew a document mirror and a projection dispatch for it, rather than a bespoke reach into the worker — the point is what a client folding the real stream ends up holding.

A first version asserted only !document.dirty, and DocumentCache::default() is already clean — so it could not tell "told it cleared" from "never told anything", and passed under an injection that stopped projecting the widget entirely. Found in re-review. It now records every marker the client is told and asserts [true, false] for the edit, with the bootstrap's own clean marker cleared first so the sequence is about the edit rather than the handshake. The stronger form fails under both injections.

a_refused_or_duplicate_add_category_moves_no_counter

improvise-75ld. App::apply_effects bumped layout_version and raised model_state.dirty off the EffectKind before the abort_effects check, so the refusing effect counted itself. Two arms, and the second is the reachable one: a 13th category is refused (MAX_CATEGORIES) and aborts, while a duplicate add-cat Region returns Ok and does not abort at all — a pure no-op that still refused every other client's next Command::Model with Rejection::LayoutMoved and made the browser worker rewrite the whole OPFS document (gotcha 41).

Injected fault: put the two bumps back on the declared kind, inside the loop (if matches!(kind, EffectKind::Model(ModelChange::LayoutGeometry))). Both no-op arms fail; so do neither_shape_counter_moves_for_an_accepted_no_op, both_shape_counters_move_for_every_reshaping_command and a_configuration_change_that_draws_nothing_is_still_a_document_change, with 450 other tests still passing — the build ran.

The positive arm is not decoration: without it, "never bump anything" passes the two no-op arms. It also fails under a second injection — dropping the document_config comparison — because AddCategory is LayoutGeometry and the declared half of dirty no longer covers it.

both_shape_counters_move_for_every_reshaping_command

improvise-pdvm. AddItem, AddItemInGroup and RemoveItem declare ModelChange::CellData and reshape the grid. Since improvise-9mo9 they call App::reset_cursor, so cursor_epoch moved while layout_version did not: a client was told its cursor was invalid while its layout basis tier still read current, and a batch composed against the old shape was still accepted.

It drives reshaping_commands() — the same table every_reshaping_model_command_invalidates_every_cursor drives, extracted into a shared function precisely so the two cannot answer for different cases — and asserts the grid shape really moved before asserting either counter, so a case that reshapes nothing fails loudly rather than proving nothing.

Injected fault: as above. Fails naming SetAxis, then AddItem once the earlier cases are removed.

a_configuration_change_that_draws_nothing_is_still_a_document_change

The third case the shape/config split creates, and the one that forced two fingerprints rather than one. hide_item records a name whether or not the model has one, and SetNumberFormat writes the file while moving no row or column: both are dirty and neither is layout_version. A single fingerprint covering the persisted configuration made a ghost hide bump the basis; one covering only the drawn shape let a format change go unsaved.

Injected fault: remove the document_config comparison in observe_reshape. Fails naming HideItem { category: "Region", item: "NoSuchItem" }.

a_reshape_of_one_view_leaves_a_client_on_another_view_alone

improvise-nynk. improvise-jytm made "which view a client is on" per-client; Sessions::adopt_cursor_epoch was not updated and still reset every mirror and returned one Broadcast::ToAll(Command::ResetCursor). A reshape of view Default therefore threw away the cursor of a client sitting on Second, which nothing reshaped.

The fixture is two views of one document with different axes, and the bystander switches to Second and scrolls there before the asker reshapes Default. The positive arm is kept in the same test — the asker is on the reshaped view, is reset, and is told — because without it "never invalidate anyone" passes.

Injected fault: drop the cursor_invalidated filter (.filter(|_| true)). Fails naming the bystander's thrown-away cursor, with 453 other tests passing.

an_unknown_reshape_set_invalidates_everybody

The fail-safe arm of the same narrowing, tested separately because it is the half gotcha 24 calls strictly worse than doing nothing. An empty reshaped set means the batch moved a cursor with no view observed to change shape; the answer must be "everybody", not "nobody", or the diff believes each client already holds origin-positioned content and sends nothing, leaving them stale permanently.

Injected fault: delete the reshaped.is_empty() || disjunct. Fails by name. That injection is why the predicate is a named function rather than a closure — a fail-safe nothing can point at is a fail-safe that gets tidied away.

a_batch_that_changes_the_layout_and_then_saves_ends_clean

A regression introduced by the first version of the observed-dirty change and caught in review of PR #70, not by the suite — the workspace was green at 1400 with this broken. observe_reshape ran after the effect loop and re-raised model_state.dirty from the document_config comparison, while Save had cleared it inside the loop. So gotcha 45's stated ordering property — "Save clears it in loop order, so a batch that edits and then saves ends clean" — survived for CellData/WholeWorkbook and was broken for LayoutGeometry.

[SetNumberFormat, Save] reported a successful save and left the document dirty: the TUI draws "Saved to …" while Quit's check refuses a later :q, and the browser worker — whose persist trigger is the flag — pays a whole-document OPFS rewrite. Reachable from :format ,.2f . w and from any Binding::Sequence pairing a geometry effect with a save.

The fix is position, not logic: the comparison moved inside the loop, taken after each LayoutGeometry effect against the value held after the previous one. Both arms are in the test — [SetCell, Save] never left the loop, so the geometry arm alone could not say whether the rule is general — and each asserts the save really happened (last_batch_aborted, and a status line starting "Saved") before asserting the flag, or a failed save would pass the dirty assertion for the wrong reason.

Injected fault: move the comparison back after the loop. Fails naming the layout arm, with 454 others passing.

The lesson worth carrying: an observation that is correct about what changed can still be wrong about when it is read. Everything above this entry argues that observing beats declaring; this is the cost of that move, and it lands precisely on the one effect that unsets what the observation sets.

the_published_basis_names_the_records_the_grid_draws_and_no_others

improvise-23qd. The set of records a Projection::Basis carries was derived twice — ClientSession::basis_update, the producer, and server_truth, a #[cfg(test)] helper that is the yardstick every fold test measures against. So a change to which records get published moved the thing and its own measure together, and every test stayed green whatever the change was. Not a helper that fabricates an input; a helper that fabricates the expected output.

Both now call published_basis. Note what that alone does not buy: the ticket's stated proof ("alter the filter and watch the basis tests fail") is impossible once one function feeds both sides — they still move together. A shared derivation removes the drift and removes the yardstick with it.

So the yardstick is spelled out rather than derived: this test names North, South and East by hand for app_with_a_reshapeable_grid's three records over six cells, and asserts that a grid where every cell is an Aggregate publishes none — because an aggregate names no record, so there is nothing to claim a version for.

Fault injected: .take(2) on the one surviving filter — the ticket's exact "change which records get published" — fails this test and nothing else in the crate. A second injection, record_version_of(..) + 1, is caught instead by the two end-to-end basis_check tests, which is the split worth knowing: the counters have other guards, the membership had none.

The fixture carries its own premise check. The first version assumed a new category lands on Axis::None; it lands on Column, and the assertion said so instead of the test quietly proving nothing.

an_aborted_batch_bumps_layout_version_for_what_it_did_run

improvise-2avl, with its negative twin an_aborted_batch_bumps_nothing_for_effects_it_never_ran. PR #70 moved the layout_version bump from "once per LayoutGeometry effect, inside the loop" to "once per batch, after the loop". Gotcha 40 records the in-loop position as load-bearing — an aborted batch must not claim effects it never ran — and the author's argument for why end-of-batch is safe for layout_version specifically (nothing ever clears it, so an end comparison sees only what ran, whereas dirty must stay in the loop because Save unsets it mid-batch) is sound and was written into context/code/command.md. An argument is not a test, and neither arm had one.

Both arms are proved independently, which is the part that earns them: bumping unconditionally fails only the second test, and skipping observe_reshape on abort fails only the first. A single injection failing both would not have shown that the two tests see different things.

Each fixture asserts app.last_batch_aborted() before asserting anything about the counter, because a batch that did not actually abort would pass both arms while testing neither.

sorting_the_store_in_records_mode_invalidates_the_cursor

improvise-ni6k names SortData as its sharpest case: it changes neither the row count nor any label, and every position still names a different record. A guard written as "did the shape change" misses it; pending_edits are keyed by row index (gotcha 1), so row 0 being West and then East is gotcha 18's misdirected-write disease arriving through a different door.

view_frame therefore hashes each record's coordinates alongside the axis entries, because records-mode row_items are positional — 0,1,2… whatever order the store is in. Fault injected: deleting those three lines fails this test and nothing else in the workspace.

The fixture trap, which is the durable part. PR #78 first shipped without the coords hash, on the stated ground that no fixture could be built where SortData reorders anything. That was wrong, and the reason is worth more than the fix: ToggleRecordsMode pushes effect::SortData as its first effect, so a records-mode fixture built by pressing R has an already-sorted store by construction and cannot demonstrate anything order-sensitive. The store itself was fine all along. So this fixture sets _Index/_Dim directly rather than going through the key path.

The test asserts the count and the labels are unchanged before asserting the cursor moved, so it cannot pass by accidentally becoming an ordinary shape change.

paging_with_nothing_pruned_invalidates_no_cursor

The negative twin of paging_across_a_prune_that_drops_a_row_invalidates_the_cursor, and the reason view_frame hashes no page selection.

improvise-7kvw is real only under prune_empty: a page flip changes which records are visible, not which categories frame them, so with pruning off a grid position keeps naming the same cell. PR #76 ruled that homing the cursor on every [/] regardless — [/] being navigation a user does constantly — is worse than the hole. Hashing the page selection does exactly that.

The seductive argument for hashing it anyway is that two pages with identical row labels really are different data. True, and irrelevant: that is record_versions' question, and view_frame answers the cursor's. A basis argument applied to a cursor decision is how this regression got written in the first place.

Fault injected: restoring the three page_selections lines fails this test and nothing else. The positive arm keeps passing either way, because with_frozen_records applies prune_empty and so already sees a dropped row in the axis entries — which is why the negative arm is the one that had to exist.

a_write_that_prunes_a_row_out_of_another_clients_view_invalidates_it

improvise-mcq4's sting, found by the adversarial review of PR #78. The mcq4 fix narrowed the frame fingerprint from every view to the active one, on the strength of a measurement showing the narrowing cost nothing. It bought its speed partly with correctness: a CellData write that prunes a row out of another client's view became invisible. view_shapes is config-only — it hashes the prune_empty flag, not which rows survive — so last_reshaped_views is empty, the one-view frame covers only the host's, and a client on the other view is never sent ResetCursor while its grid loses a row underneath it. That is gotcha 24's "strictly worse than doing nothing": the diff then believes the bystander is current, so it stays stale permanently.

The repair keeps the narrow one-view frame on the LayoutGeometry arm — the held-key path improvise-748f exists for, where other views cannot reframe because view_mut() writes only the active one — and restores the all-views frame on the CellData arm. Measured: 0% on the held-key path, +5.7% on an 18k-cell two-view SetCell, scaling linearly in view count.

Writing this test found a second bug, which was masking the first. The precondition failed at first: grid_in built a non-active view with GridLayout::new, which does not apply prune_empty, so a client on another view was projected three rows where its own config gives two. Their grid never pruned, so it never lost a row, so the staleness was unobservable — a bug hiding a bug. It is the third instance of the new-versus-with_frozen_records trap (view_frame was the second), and fixing it broke no other test.

Fault injected: removing the all-views arm fails this test, plus sorting_the_store_in_records_mode_invalidates_the_cursor (SortData is CellData) and both cursor-audience tests — four in all.

a_switch_then_a_geometry_effect_compares_within_one_view

Found by the same review. frame_basis was taken once at batch open against view_state.active_view; SwitchView is ViewOnly, so it did not re-base, and the next model effect hashed the new view and compared it against the old view's hash. A near-certain mismatch, so a spurious reset_cursor() and a wrong name in last_reframed_views.

Today's instances are benign — DrillIntoCell and EnterRecordsMode both emit [CreateView, SwitchView, SetAxis, …] in one batch, but against a view they just created. The harmful case needs a switch to an existing view with a saved cursor plus a model effect in the same batch, which a .-chained : line reaches. Low severity, unsound mechanism.

The basis is now keyed by view name and re-bases rather than comparing when the name moves. Fault injected: dropping the was == self.view_name() guard fails this test, with cursor_epoch moving for a batch that reframed nothing.

improvise-9c9o moved where the re-base happens, and this test was unchanged by it because it names the property, not the mechanism — the re-base now fires on the switch rather than on the next geometry effect, and a batch that reframes nothing still invalidates nothing.

Its fixture carried a line that did nothing at all: views["Second"].clone_into(&mut views["Default"].clone()) clones into a temporary that is immediately dropped. It read as the thing making the two views differ, so had create_view ever started copying the active view's config this test would have gone vacuous with that line standing there looking like it prevented exactly that. It is deleted and replaced by the assertion it was pretending to be: the two views' view_frame hashes really differ.

a_geometry_effect_after_a_mid_batch_switch_still_invalidates_the_cursor

The other side of the guard above, and introduced by it. PR #78 round 1 compared frame hashes across a view change, which threw a cursor away spuriously; round 2 added was == self.view_name(), which fixed that and also exempted the first geometry effect after a mid-batch SwitchView from the check altogether — there is no basis for the new view yet, so the code re-based and skipped. Passes on pre-#78 main and on round-1 head; fails at the merged head.

Direction is why this is worth more than the bug it replaced. Round 1 kept a cursor it should have thrown away; round 2 threw away nothing it should have kept — which is gotcha 35's expensive side, since a stale cursor has nothing in the stream to correct it where a spurious invalidation costs one projection.

The fix is the ticket's: the basis follows active_view, re-based in the loop as soon as the name moves, rather than at the next geometry effect. It did not need SwitchView to stop being ViewOnly and it is not a protocol change — the basis is App's own bookkeeping, and nothing about it is on the wire. The was == self.view_name() guard is gone from the comparison because it is now unreachable: by the time the geometry arm runs, the name and the basis agree by construction.

What it changes elsewhere, and why that is fine: DrillIntoCell and EnterRecordsMode both emit [CreateView, SwitchView, SetAxis, SetAxis, …], so they now reset the cursor on the first SetAxis where they used to skip it and reset on the second. Same outcome, one effect earlier. DeleteView is the one LayoutGeometry effect that moves active_view itself; the re-base fires for it too, so the geometry arm compares the new view against itself and resets nothing — which is what the old skip did.

The fixture measures the row count the transpose must move rather than spelling it: a first version asserted row_count() == 1 and the real answer is 2, which is the shape of a precondition that would have gone quietly wrong later. It fails on the unfixed tree at the epoch assertion, left: 0 right: 0.

a_frozen_drill_does_not_take_its_opening_frame_from_the_held_layout

improvise-mcq4's fix reads the batch's opening frame off App::layout rather than building a second pruned layout — that is where the 4.1×→1.9× came from. It is guarded on drill_state.is_none(), because a drill freezes the record list and the held layout is then not what view_frame would build for that view.

The guard was correct, load-bearing, and tested by nothing: reverting it to the unconditional form left all 1439 tests passing. Textbook doctrine(a_careful_comment_is_weak_evidence_that_anything_checks_it), and it was the most carefully argued comment in the PR.

The fixture freezes one record where the live query finds three — asserted, so a freeze that did not narrow could not pass — then runs a SetNumberFormat batch, which reframes nothing. Without the guard the opening basis is the frozen layout and the in-loop hash is the live one, so they differ and the cursor is reset for a batch that changed no row. Fault injected: replacing self.drill_state.is_none() with true fails this test and nothing else in the workspace.

a_clients_grid_does_not_depend_on_which_view_its_host_is_on

improvise-t6db. The one-line grid_in fix landed in PR #78 with no test of its own; this is the general property the reviewer specced — host-view independence: grid_in(app_on_A, "B", vp) == grid_in(app_on_B, "B", vp) for every ordered pair of views. It needs no second derivation of what a view's config means, which is why it is a property and not a second copy of GridLayout.

The trap is the fixture, not the assertion. The reviewer's first version made the two views differ by turning prune_empty off in the second — plenty of differing config, and vacuous: with nothing to prune, GridLayout::new and GridLayout::with_frozen_records agree and the property passed on the buggy tree. So app_with_three_views_and_an_empty_row gives Service no cell at all and leaves all three views pruning, and the test asserts every view really is two rows before comparing anything — a fixture that stopped pruning would fail there rather than passing quietly.

The property is stated with drill_state.is_none() asserted, because a frozen drill snapshot is the one thing grid_in cannot reproduce for a view the host is not on (with_frozen_records(.., None) on the elsewhere path).

Fault injected 2026-09-21: restoring GridLayout::new in grid_in — the pre-#78 line — fails this test at the assert_eq!, not at the precondition. It also fails a_write_that_prunes_a_row_out_of_another_clients_view_invalidates_it and a_hosts_none_cats_leak_into_another_views_formula_columns: three tests, not the one an earlier draft of this entry claimed. The #78 fix has more coverage than it was credited with.

The none_cats asymmetry is REAL, and the first draft of this entry reported it as unobserved from a fixture that lacked the condition. That correction is worth more than the test. The claim was that the fixture put Product on None in Default and on Column in Second; it did not — add_category defaults a new category to Column and nothing moved it, so all three views had none_cats {_Index, _Dim}, the stated input to the hazard was absent, and Default and Second were indistinguishable to grid_in apart from a number_format that is itself a no-op. An unprobed path reported as a negative result, which is doctrine(an_injection_that_does_not_fail_names_an_untested_path) applied to a setup, and the expensive kind: it stops the next person looking.

Setting Product to Axis::None explicitly made the property fail on the first run, and not on the value of a cell — Second lost both Total columns entirely when the host stood on Default. Filed as improvise-ac2z, pinned by the characterization test below, and the property now skips pairs whose none_cats differ. That skip is a bug being worked around, not a carve-out; it asserts compared > 0 so a future fixture cannot skip everything and pass.

a_window_narrows_the_cells_and_neither_the_totals_nor_the_widths

improvise-jmjz. project::grid_of evaluated the whole grid roughly three times per frame — compute_col_widths walked every cell for its text and then every cell again for the column totals it must size, and grid_of then evaluated the window a third time. ui/viewport.rs::grid_body now evaluates each cell once and derives the widths, the totals and the window from that value.

That fusion creates a hazard the three separate passes did not have: one walk now serves three consumers with different scopes. Two faults follow, not one — a window that leaks into the totals makes a column total the sum of the visible rows, and a window that leaks into the widths sizes a column by what happens to be on screen. Neither is visible from the output: the numbers simply get smaller, consistently.

The fixture took three attempts, and what each one was blind to is the entry.

First: rows of 1, 20, 300000 under a measure named Revenue, asserting col_widths[0] > "300021".len(). It could not see a windowed width at all — Revenue is 7 characters against a 6-character value, so the assertion passed on the header alone. Worse, its other half was structurally incapable: comparing grid_body's widths against compute_col_widths cannot test windowing, because compute_col_widths is grid_body with an empty window. Windowing degrades both sides identically, both collapsing to the MIN_COL_WIDTH clamp. The same function twice is not a check, and it reads exactly like one. Measured by the PR #92 review: if drawn { widen(..) } left 1476 passing, 0 failing.

Second, the reviewer's: 1, 888888888, -888888888 with a one-character measure name, so the total cancels to 1.0 and neither it nor the header can supply the width. This fixed the width half and broke the other: the windowed row is 1.0 and the cancelled total is also 1.0, so the totals assertion went blind to the row-window fault, which the widths assertion then happened to catch. Two assertions, one no longer discriminating, and nothing saying which.

Third, and current: the same values with the large ones first, so the window holds 888888888 while the total is 1.0. Each injection now fails on the assertion that names it:

• for ri in rows.clone() — the whole walk windowed → totals [888888888.0] against [1.0]
• if drawn { widen(..) } — widths only → widths [10] against [11]

10 rather than 5 is a smaller gap than the reviewer's fixture gave and a better witness: the missing character is the minus sign, which only -888888888 carries and the window does not contain. The test also asserts the window really is narrower than the grid, so a fixture that stopped windowing would fail at the precondition.

What this test does not do is prove the speedup, and no test can — the outputs are identical by construction, which is why the whole suite passed unchanged across the change. The evidence is the benchmark, before and after, in context/code/command.md: about 3x, the number of passes removed.

a_formulas_columns_survive_a_host_that_aggregates_differently

improvise-ac2z, fixed. This replaces the characterization test a_hosts_none_cats_leak_into_another_views_formula_columns, whose message said that failing meant the bug was fixed; it did, and it is gone.

App::rebuild_layout calls recompute_formulas(&none_cats) for the host's view, populating Model::formula_cache. project::grid_in on the elsewhere path built the other view's GridLayout against that same cache, which is keyed by the coordinates the recompute used: with Product on None in the host's view the Total cells are keyed without a Product coordinate, a view with Product on Column asks for (Region, Product, _Measure=Total), finds nothing, and — prune_empty being on — drops both Total columns. Not a wrong number: absent columns.

host on Second, projecting Second 6 columns, Total present host on Default, projecting Second 4 columns, no Total at all

The fix is grid_in::rederived_for: where the target view aggregates over different categories than the host's, it clones the model and recomputes the formulas for that view's none_cats. Three conditions gate the clone — another view, different none_cats, and at least one formula — so the common case pays nothing and a formula-free document pays nothing ever.

This is an interim, and the cost is the reason. A clone of the whole model plus a fixed-point recompute, per projection, on exactly the path that is broken today. The ticket's first candidate — keying formula_cache by none_cats so one Model can hold more than one view's derived values — is the durable answer and is filed separately; it is an improvise-core change and did not belong in this wave's ground. What is bought meanwhile is correctness on a path that was silently wrong, doctrine(the_elsewhere_path_is_only_as_correct_as_the_inputs_it_can_rebuild).

Fault injected: rederived_for's condition forced to false. Two tests fail — this one, and a_clients_grid_does_not_depend_on_which_view_its_host_is_on, whose none_cats carve-out was removed at the same time and which is the general property this one is an instance of. The test asserts its two preconditions first — that Second on its own really draws the formula's columns, and that the two views really do aggregate over different categories — so a fixture that lost either fails rather than passing on two empties.

a_view_named_command_lands_on_the_view_it_names

improvise-lltt, the oldest open P1 here, opened by improvise-jytm making ViewState::active_view authoritative per client.

Nine ModelCommand variants configure a view — SetAxis, CycleAxis, TransposeAxes, SetPageSelection, HideItem, ShowItem, ToggleGroup, TogglePruneEmpty, SetNumberFormat — and each named its view implicitly: model_effects built an effect that wrote App::view_mut(), which is the host's slice. A client that had switched to Second therefore moved Default: its own grid did not change, and every other client's did.

The fix is the protocol change the ticket asks for. Each of the nine carries a view: String, model_effects maps it to ViewTarget::Named, and the nine effects carry a ViewTarget rather than reading an ambient one. There is no operandless form left: App::view_mut() is deleted, replaced by view_mut_of(&ViewTarget), so a future geometry effect has to say which view it writes. The host's own key path passes ViewTarget::Active, which is right because the TUI is the host and has no session to ask.

Fault injected: model_effects' SetAxis arm rebuilt with ViewTarget::Active (the pre-fix behaviour, with view bound and discarded so it still compiles). The test fails on the asker's own view never moved — Second's Channel stayed on Row while Default's moved to Page.

The fixture must seat the asker somewhere the host is not, and the two views must disagree about the category being moved: Channel is Axis::None in Default and Axis::Row in Second, and the command asks for Axis::Page, so both assertions separate the fixed code from the broken code in both directions.

a_reframe_of_a_view_the_host_is_not_on_invalidates_its_clients

improvise-lltt's second half, and the part the ticket did not predict. Naming the view made a reframe of a view the host is not on reachable for the first time, and Sessions::adopt_cursor_epoch could not see one: it returned early unless App::cursor_epoch had moved, and the epoch is bumped only by reset_cursor, which apply_effects calls only for the host's own view. So the clients on the reframed view were reset by nobody and told nothing — gotcha 24's strictly-worse-than-nothing state, permanently stale rather than stale until the next scroll.

App::last_reframed_elsewhere is the second trigger. Its opening frames are taken lazily inside App::view_mut_of, which is the one door such a write goes through, so the held-key path still hashes exactly one view: the wide-observation cost improvise-mcq4 declined is not paid.

Fault injected: the gate restored to if !host_moved { return Vec::new(); }. The test fails on a client on the reframed view was never told.

Two negative arms, and they are not decoration. A third client left on Default must keep its offset, or "invalidate everyone" passes; and paging_with_nothing_pruned_invalidates_no_cursor is what forbids relaying against last_reframed_views, which is a union with the reshaped set and so fires for a page selection that pruned nothing. That test failed during development and is why the elsewhere set is precise.

a_command_naming_a_view_the_document_has_not_got_writes_nothing

App::resolve_view falls back to the document's default for a name the document has not got, which is right for reading — a slice naming a deleted view must not panic in rebuild_layout — and wrong for writing, where it would move a view nobody asked about. view_mut_of therefore looks the name up directly and returns None.

Fault injected: view_mut_of's Named arm routed through self.resolve_view(name).0. The test fails on a name the document has not got fell back onto a view that has one.

The window this closes is narrow — Sessions::adopt_deleted_views already moves a mirror off a deleted view and tells it — but it is a race, not an impossibility, and the silent alternative is a write to the wrong document state. Not closed here: the client hears nothing. A Rejection variant for it is filed rather than smuggled in, since Rejection is wire vocabulary.

the_diff_tells_a_client_the_grid_became_a_records_grid

The producer half of GridCache::records (improvise-e5pf). grid_of setting the field is exercised by leaving_a_panel_in_records_mode_lands_where_the_host_lands; the diff is a separate arm, and a client that connects before records mode is entered learns only through it.

Both preconditions are asserted — the fixture starts on an ordinary grid, and R really reaches a records one — so the comparison is between two answers that had something to disagree about.

Fault injected: the records arm deleted from grid_updates. The test fails on the client is never told. The negative arm (an unchanged flag is not resent) is what stops "always push it" from passing.

a_batch_that_leaves_records_settles_out_of_records_normal

improvise-4dpj, gap 1. settle_normal_mode matches Normal | RecordsNormal, and replacing that with a one-way AppMode::Normal left the entire workspace suite passing. The existing crossing test, a_batch_that_leaves_records_and_then_sets_normal_lands_in_normal, drives the layout both ways, but both of its branches end in enter-mode normal, so the mode handed to the settle is AppMode::Normal in both directions. It discriminates "settles at all" and "settles against the ending layout", and not "settles from either mode".

This drives the production repro: from a records grid, apply_effects([SetAxis { "_Index", Axis::None }]) leaves layout.is_records_mode() false with the mode still RecordsNormal, so the RecordsNormal arm is live and not defensive. Both the records-ness of the grid and the mode before the batch are asserted as preconditions, so a fixture that stopped entering records mode fails loudly rather than proving nothing.

Injected fault: if matches!(self.view_state.mode, AppMode::Normal). Fails by name, left: RecordsNormal, right: Normal. It is the only test in the workspace that fails.

switching_views_across_records_ness_settles_against_the_view_switched_to

improvise-4dpj, gap 2 — raised by ChatGPT Codex against PR #91 and verified correct on main before being tested. SwitchViewAtCursor emits [SwitchView(target), ChangeMode(Normal)], and this is a different path from the one gap 1's siblings drive: that test toggles records mode explicitly, where a view switch crosses records-ness only because it is auto-detected from the axes (gotcha 1). Nothing in the batch says "records".

Driven in both directions, with the crossing asserted before and after so a fixture that stopped crossing fails rather than passes. Injected fault: move settle_normal_mode() back above rebuild_layout() at the end of apply_effects — the exact regression PR #91's round 2 fixed. This test fails by name; so does gap 1's, along with six pre-existing records-mode tests (measured in review; an earlier draft of this entry said five).

Scope note, stated because an overclaim is worse than a gap: this drives the effects SwitchViewAtCursor emits, not the view-panel key path that composes them. Reaching the panel needs a view_panel_cursor on a CmdContext built in command/cmd/panel.rs, which is another wave's ground this round. The command's own coverage lives there; what is unexercised is the two joined.

the_layout_geometry_cases_are_every_wire_effect_of_that_kind

improvise-26dm. layout_geometry_effects_actually_change_document_config was guarded by assert_eq!(cases.len(), 13), which catches a case being deleted from the list and never one being absent from it — and the two are written identically as an assert_eq over a number. The adversarial review of PR #78 added a fourteenth pub struct declaring LayoutGeometry, pointed at by nothing, and the whole suite stayed green.

improvise-5ubs's mechanism cannot reach it: an effect is a struct implementing a trait, so serde has no derived enumeration to diff against. model_effects maps each ModelCommand variant back to the effect it came from (gotcha 38), so the list is diffed against the LayoutGeometry effects reachable through ModelCommand. The two sets are equal today, which is the evidence the substitution lost nothing.

The right-hand side is bounded by the derive, and that took a second round. Review of PR #96 injected the deletion of ModelCommand::TogglePruneEmpty from every_model_command(): this was the only test in the workspace that failed, and every_model_command's doc comment claiming the list was compiler-forced was wrong — variant_name's exhaustive match forces an arm there, not an entry in the list. ChatGPT Codex raised the same thing on the PR. the_enumeration_names_every_variant_the_derive_declares closes it, so the chain is derive → enumeration → model_effects → case list with every link checked. The assertion here still names both directions, because a short right-hand side reads as a spurious case on the left and invites deleting it — degrading both sides together.

Injected fault: drop TogglePruneEmpty from layout_geometry_cases(). The old count said 12 != 13; this says right: [… "TogglePruneEmpty" …] and names the effect.

Residual, stated rather than closed: this is total over the wire and not over the effects. An effect declaring LayoutGeometry that no ModelCommand names is still invisible — which is precisely PR #78's probe. Closing that needs an enumeration of effects the compiler maintains, and none exists without a proc macro or inventory-style registration.

the_enumeration_names_every_variant_the_derive_declares

The bound the entry above rests on. Injection is Codex's own, run end to end: a ModelCommand::ProbeGeometry whose effect declares LayoutGeometry, with the model_effects and variant_name arms the compiler forces, omitted from both lists. Before this test, the whole workspace stayed green. After it, the failure is staged and each stage names the probe — first every_model_command() never constructs ["ProbeGeometry"], then, once the enumeration is repaired and only the case list is short, the_layout_geometry_cases_are_every_wire_effect_of_that_kind with ProbeGeometry on the right. That staging is the point: the two tests answer different questions and the injection separates them.

The name reader is shared, not copied: reduce.rs's effect_type_name and its own the_effect_name_reader_reads_all_three_debug_shapes are what this leans on, because a second reader here would be a derivation done twice and could drift while one copy was repaired. It reads the leading identifier off the derived Debug, Effect being a trait object that type_name cannot reach.

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command/cmd/panel.rs — the TUI half of the category-panel defects

Both of these have a twin in improvise-protocol/src/resolve.rs whose leaf name is identical, so both headings are module-qualified (gotcha 48). Neither defect was browser-specific: the same Cmd backs the same key in the TUI, and confirming that before fixing is what kept the fix out of the client.

command::cmd::panel::tests::deleting_a_categorys_last_item_leaves_the_cursor_inside_that_category

improvise-uej8 in the TUI. The full argument, including why the rival "always step back" repair is worse than the bug, is under resolve::tests::deleting_a_categorys_last_item_leaves_the_cursor_inside_that_category in clients.md; it is not repeated here, because two copies of an argument are two things to keep true.

What this test adds that its twin cannot: the TUI tree carries the three virtual categories ahead of the regular ones (improvise-vjms, gotcha 7), so its indices are the ones a real user's cursor holds. Row 5 is Type's last item and row 6 is Month's header; the test asserts both before acting, so it cannot pass by landing somewhere the escalation was never possible. Inject the fault by deleting the effects.extend(retreat_from_deleted_item(...)) line.

command::cmd::panel::tests::deleting_an_item_with_a_sibling_below_it_leaves_the_cursor_alone

The discriminating input for the rival fix, in the TUI. Fails under an unconditional step-back, passes under the conditional one.

command::cmd::panel::tests::deleting_a_categorys_only_item_does_not_arm_the_next_d_with_remove_category

The singleton arm in the TUI. The enumeration and the landing argument are under the resolve.rs twin of this name in clients.md; they are not repeated here, because two copies of an argument are two things to keep true.

What this fixture adds: Month is seated before Type and carries two items, so the safe landing is found by the backward search — the preferred direction — and lands on a real item rather than falling off the end of the tree. Its twin exercises the same case with nothing below. Both assert the consequence (a second d composes no RemoveCategory) rather than the landing index, and both fail under the pre-review rule while the two adjacent-case tests pass.

command::cmd::panel::tests::cycling_an_axis_from_the_category_panel_says_so_as_the_tile_bar_does

improvise-25bf in the TUI. The category panel and the tile bar are two routes to one operation and only the tile route told the user, which is design-principles §5 in miniature — a derivation done twice will diverge, and the divergence is invisible until someone puts them side by side. The wording now has one home, viewmodel::axis_change_status, called by both routes here and by both in the browser.

The fixture sets Type to Row explicitly rather than reading the default, so the expected Type → Col is a literal and not a restatement of the code under test. It asserts cat_at_cursor() is Type first, because cursor 3 is only the first regular category while the three virtual ones lead the tree — an off-by-one would otherwise pin the wrong category's sentence and still pass.

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The view stack is the client's (improvise-gd28)

Two user rulings, 2026-09-21: "The state drill manipulates (the view stack) should be local to the client" and "Drilling should be an ephemeral view, not something that gets persisted." The first one is what these three tests serve.

a_client_folding_a_switch_records_the_history_the_host_does

The bug, demonstrated before the fix. effect::SwitchView pushes the view it is leaving onto ViewState::view_back_stack and clears the forward stack; reduce_view's Command::SwitchView arm set active_view and nothing else. Both write the same portable field of the same type, and the fold is supposed to be the mirror's copy of the effect (improvise-cqq/m91) — so a connected client's own history was empty forever, however many views it visited.

That is the whole reason view-back reads as unavailable in a browser: not that stepping back is hard, but that the client has no stack to pop. The declined() comment attributes it to the drill commit instead, which is a second, separable reason (see the drill note below).

Written against the unfixed tree it fails with left: [] right: [ViewFrame { view_name: "Default", mode: Normal }]. It drives the host through apply_command and the mirror through reduce_view, so it compares the two writers rather than one writer against itself.

a_client_stepping_back_lands_where_the_host_lands

The lockstep for the two new commands. effect::ViewBack restores frame.mode as well as the view name, so a fixture left in Normal cannot tell "restores the mode" from "never touched the mode" — the mode the default supplies is exactly the mode a broken restore leaves behind. The fixture therefore enters Help, switches away, drops to Normal, and asserts Help comes back. Deleting the view.mode = frame.mode line fails it; deleting it from a Normal-only fixture would not.

stepping_forward_undoes_stepping_back and a_step_off_the_end_of_the_history_moves_nothing bound the two ends: the direction is a Rewind enum rather than a bool so the two arms cannot read each other's stack, and an empty stack is a no-op rather than a panic.

a_step_back_cannot_land_on_a_view_the_document_has_lost

The case the client-local stack creates, stated where a user feels it: a frame on my stack can name a view somebody else deleted while I was away.

This test replaced a guard. The first version answered it inside Sessions::apply_step, which folded the step and then corrected a dead landing the way apply_switch corrects a refused switch. Two injections did fail it — but the correction folds a SwitchView, which pushes the dead view straight back onto the stack, so pressing < again popped it, was corrected again, and pushed it again: a back button that never progresses. Pruning the history when the view vanishes (see the entry above) makes the dead frame unrepresentable instead, which left apply_step's guard with no reachable input. It was deleted rather than kept as defence in depth, because a guard whose fault cannot be injected is indistinguishable from one that always passes, and apply_checked's existing fold arm is then the whole behaviour.

The fixture visits Third and then Second, so the stack is [Default, Third, Second] and only the top frame is doomed. Landing on Third is what discriminates: a fixture whose stack held only the dead frame would land on Default and could not tell "skipped it" from "gave up and went home". Injected fault: drop forget_vanished_views from apply_model — the step lands on Second, a view the document has not got.

deleting_a_view_forgets_every_frame_that_names_it

A defect the client-local stack creates, and a pre-existing one on the host. A ViewFrame is somewhere you can go back to, so a frame naming a view that has been deleted is an offer the document cannot honour. effect::DeleteView moved active_view off the doomed view and left both stacks alone, which was invisible while no client had a stack at all — the TUI user could already reach a dead frame with <, and making reduce_view record history would have given every browser client the same.

Worse than merely dead: without the pruning, Sessions::apply_step corrects the landing by folding SwitchView(default), which pushes the dead view back onto the stack. Pressing < again pops it, is corrected again, and pushes it again — a back button that never progresses, and one that looks from the outside like a key that does nothing.

Injected fault: delete the forget_view call from effect::DeleteView. The test names the surviving frame. Its fixture asserts the doomed view is on the stack before the delete, so it cannot pass by pruning nothing from nothing.

a_deleted_view_leaves_the_history_of_a_client_that_did_not_delete_it

The multi-client half, and the one adopt_deleted_views cannot cover: that helper looks at mirrors sitting on the deleted view, and a client that merely visited it is sitting somewhere else entirely. The bystander here is therefore on Third, holding Second on its back stack — which is also what makes ToAll the right audience and checkable: every client may have been there, and the asker is not special.

Which views vanished is observed by diffing the name set across apply_command, for the reason adopt_cursor_epoch watches a counter and apply_model watches document_generation (gotchas 24, 39): which effects drop a view is a per-effect decision, so a table of command variants would be a second description of it, free to disagree.

Injected fault: drop the forget_vanished_views call from apply_model. The bystander keeps its dead frame and the test names it.

a_refused_switch_puts_no_phantom_on_the_history

Found by ChatGPT Codex on PR #115, and it is that PR's own regression. Once reduce_view records history, the correction for a refused switch records history too. Starting on Default, the asker folds Missing (back gains Default), then folds the correction back to Default (back gains Missing) — while the mirror, which never folded either, stays empty. So < afterwards lands on a view the document never had, and forget_view cannot help: nothing vanished, the name was never real.

It is reachable, which is why it was fixed rather than argued away. No shipped client can invent a view name — resolve.rs composes SwitchView from a ViewSummary in the projected list — but a client can pick a name that another client deletes before the frame arrives. That race is exactly the case forget_vanished_views narrows and does not close.

Two things the fix needed, and the test separates them:

• The mirror folds what the client folded, refused or not. A correction can only undo what both ends hold; guarding the mirror is what let the two drift.
• The correction is [ViewBack, ForgetView(asked)] — the exact inverse of the push, then the refused name off the forward stack it lands on.

Injections, each failing for its own reason:

• Drop ForgetView: the phantom assertion fires — Missing is on the history.
• The rival fix, SwitchView(leaving) instead of ViewBack (which is what the review's own wording suggests, and what the code did before): no phantom, but left: [Default, Second] against [Default] — the redundant frame the correction pushed. That injection is the reason the test asserts the back stack is restored rather than merely free of the phantom.

One residue, asserted rather than hidden: the optimistic fold clears the forward stack and no correction can refill it, so a refused switch costs the redo history — exactly as an accepted one does. The test pins it empty, and the fixture loads both stacks first so a lost forward stack is visible at all.

The neighbouring test a_refused_switch_corrects_the_asker_and_nobody_else compared active_view alone and stayed green through all of this: the check that answers a neighbouring question, in the place the doctrine warns about.

command/keymap.rs — a bound key that builds nothing (improvise-55l5)

a_bound_key_whose_command_cannot_be_built_says_so

dispatch resolved a bound name with registry.interactive(name, args, ctx).ok()?, so a command that would not construct collapsed into the same None an unbound key returns — and App::dispatch_key answers None with Dirty::NOTHING and no status. The two are different facts and the option could carry only one of them.

Which errors were invisible, and what each would have caught:

• no cell at cursor — six registrations read ctx.cell_key(): clear-cell, yank-cell, paste-cell, drill-into-cell, commit-cell-edit and commit-and-advance-right. This is the class that reaches a user: it is state-dependent, so no compile-time or registration check can see it. x on a grid whose row axis carries an empty category did nothing and said nothing.
• Unknown command, require_args, Unknown mode, commit-buffer item requires item-add mode — keymap-authoring errors. every_bound_command_name_is_registered already refuses these at test time (gotcha 15 checks both halves for exactly this reason), so they are unreachable from a shipped keymap. They surface now because the mechanism is one rule rather than a list, not because they were the complaint.

The repro is the first class. app_with_no_cells puts an item-less category on Axis::Row, and the setup asserts layout.cell_key(0, 0) == None — so the fixture states the precondition rather than hoping for it. The test then asserts x is bound before pressing it, because a test that pressed an unbound key would pass against the bug.

Injected rivals, one per test:

• Some(built.unwrap_or_default()) — return Some but swallow the error, the smallest thing that looks like a fix. a_bound_key_…_says_so fails; an_unbound_key_stays_silent passes, as it should.
• Report a failed lookup as well (None from Keymaps::lookup becomes a status). an_unbound_key_stays_silent fails; the other passes. That is the neighbouring question this pair separates: did a binding match is not did the command build, and a fix that answers both with a message is noise on every typo and every unmapped key.

A Sequence whose later step fails now reports and runs none of it, which is the existing all-or-nothing shape (try_fold), not a new rule — gotcha 21's abort is about effects that ran, this is about effects that were never built.

an_unbound_key_stays_silent

The control. None still means, and only means, no binding matched. It presses a key the Normal keymap does not bind and asserts the status stays empty, and it asserts that key is unbound first — without that assertion the test would pass against a keymap in which it was bound to something silent.

command/cmd/grid.rs — o edits the row it added (improvise-o9uq)

o_in_records_mode_edits_the_row_it_added_and_leaves_the_others_alone

o is [add-record-row, enter-edit-at-cursor records-editing], and AddRecordRow::execute returned only SetCell + a status: it never placed the cursor. Appending a record changes view_frame's hash, so the frame observation inside apply_effects (gotcha 24) invalidated the cursor to the origin mid-batch, and EnterEditAtCursor then opened an editor pre-filled with row 0's Region coordinate. Typing appended to it and Enter went through commit_plain_records_edit → rename_record_coordinate, moving every measure cell of an unrelated record (gotcha 1: a category-column edit moves the whole row, which is why the basis unit is the record — gotcha 40).

Measured before the fix, from Region{East,West} with Revenue 10 and 20, at sel=(1,0), keys o N o r t h Enter: East's Revenue cell was gone and the rows read ["0","1","2"] — the East record had become EastNorth.

The fix appends viewport_effects(target_row, 0, ..) after the SetCell, which is the whole content of the repair: the observation is inside the loop, so a placement emitted after the reframe survives and one emitted before does not.

Injected rival: move the viewport_effects ahead of the SetCell. Both tests fail — the reframe then overwrites the placement, which is the exact bug. That rival is the one worth keeping, because the naive reading of the acceptance criteria ("append viewport_effects") does not say where, and the wrong order is indistinguishable from the right one by inspection.

target_row is row_count() — the appended row is last — unless the record already exists, in which case it is that record's own index, found by scanning record_row(r).coords. Predicting row_count() unconditionally would put the cursor out of range in that arm.

o_in_records_mode_opens_the_editor_on_an_empty_coordinate

The same defect one step earlier, and the cheaper witness: after o alone, the edit buffer must be empty. Pre-filled with another row's coordinate is the state from which any subsequent keystroke corrupts. It is a separate test because it fails without needing the commit path to run, so a regression in placement is diagnosed without reading through rename_record_coordinate.

Two things this fix does not close, deliberately:

• o twice with no page-axis category still adds one row. Both presses build CellKey::new(vec![]) — the same record — and the second now says so rather than claiming "Added new record row". Refusing outright was rejected: CommitAndAdvance's Tab-at-bottom-right path calls the same command and relies on seeding an empty-coordinate row.
• 1b proper. view_frame hashes entries.len() and every entry, so any append reframes and invalidates the cursor although no existing row was renumbered. Placing the cursor masks it at this call site; distinguishing "appended" from "renumbered" is a separate question and stays filed.

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The format a projected cell is drawn with (improvise-e0ex)

a_formulas_format_reaches_its_own_column_and_no_other

context/code/core.md recorded, as the more useful half of improvise-711n's entry, that ui/project.rs::grid_of "has no test that renders a cell and asserts its digits". This is that test, and it is why the gap was worth naming: the per-formula format is resolved in improvise-core and could have been resolved correctly there while never reaching the grid.

It asserts four things that fail independently: the formula column's cell, the raw column's cell beside it, and both totals. The totals matter separately because they are formatted on their own path — GridBody::col_formats, read off the top row — so a correct cell says nothing about the strip under it.

Two faults injected:

• FormatPlan::at returning the view's format for every key fails the cell assertion (left: Some("0.50") right: Some("0.5000")).
• Dropping only the col_formats write in viewport::grid_body, leaving the cells correct, fails the totals assertion (left: "1.50" right: "1.5000").

The second is the one that earns the test its length: without it, the totals strip could round every formula column to the view's format and no assertion in the workspace would move.

a_column_total_does_not_inherit_a_row_local_format

Found by review (ChatGPT Codex, PR #125). The first version of grid_body read each column's total format off row 0, and context/code/core.md argued that was safe because "where _Measure is on the row axis a column is not a measure at all". That argument is wrong: a formula's target category is any category, so a formula can own a row-axis item, and then row 0's format is a row-local override applied to a total that has summed that row away — and re-ordering the rows changes the total's spelling.

The fixture builds the same grid twice with 12.5 FORMAT .4 owning the first row in one and the last row in the other, and requires the two totals strips to agree. Every raw cell in raw_workbook holds the same value per measure, so the two grids' totals are numerically identical and the only thing that can differ is the format — the assertion has nowhere to hide.

It asserts the owned cell first (12.5000), so a plan that applies no format at all fails on the fixture rather than passing the comparison vacuously.

Fault injected: plan.at(layout.cell_key(0, ci).as_ref()) in place of layout.column_key(ci) — the rejected version, restored. Fails with left: ["14.5000", "16.5000"] right: ["14.50", "16.50"], which is the bug in one line: row order decided the spelling.