improvise/context/code/clients.md

# Clients: the browser client, the two hosts, and the harness

The arguments that used to live in the comment headers of
`improvise-wasm-client`, `improvise-ws-server`, `improvise-worker-server`,
`web/` and `harness/`. Written for a reader with the code open. Bug narratives
keyed by test name are in `context/test-evidence/clients.md`.

Repo-map gotchas 42, 44, 45, 46, 47 and 48 cover the same ground from the
"where" side; this file is the "why", at the density the comments had.

---

## `improvise-worker-server/src/worker.rs` — the message loop

`Worker` is the whole browser host minus the two things a browser has to
supply — `self.onmessage`/`self.postMessage`, and OPFS. Both are in
`crate::bindings`, which is `cfg(target_arch = "wasm32")` and has no decisions
in it. Everything that could be *wrong* about what the main thread is told is
in `worker.rs`, where `cargo test` can drive it.

### It is the socket's `Host`, not a second one

`Host` is `App` + `Sessions` + `handle(id, ClientFrame) -> Vec<Addressed>`.
Nothing in it is a socket, and improvise-9x6 is what made that load-bearing
rather than tidy: the worker needs the same session loop and cannot depend on
`improvise-ws-server`, which is tokio and tungstenite. So `Host` moved down to
`improvise_command::ui::host` and **both deployments name the one type**.
Re-implementing it here would have been a mirror, and improvise-xks6 had just
finished deleting the last one: a second copy of a decision layer drifts, and
the drift is invisible until a client sees it.

What the worker adds on top is only what a `MessagePort` differs in:

- **One client, by construction.** A `MessagePort` is point to point, so the
  `SessionId` is not an address the transport has to carry — it is a field.
  `Addressed` is still honoured rather than assumed away (`Worker::frames_for`),
  because a `SharedWorker` would make it matter again and the honest filter
  costs one line. `Host::relay` addresses a broadcast to every session it
  knows, and with one session that is always this one — a loop that assumed it
  away would be silently wrong at that point rather than obviously incomplete.
- **A handshake with no close.** `ClientFrame::Viewport` must come first, for
  the reason it must on the socket: a `Snapshot` seeds the *visible* cells, so
  one built at extent zero is an empty grid. A socket enforces that by closing
  the connection; a worker has nothing to close, so a frame that arrives before
  the handshake is **dropped**, which is the same outcome without a mechanism
  that does not exist here.

### `ClientFrame::Key` is answered with nothing, and that is not laziness

A worker holds the authoritative `App`, so `ui::reduce::reduce_key` really is
callable here where it is not on a socket. Calling it would still be wrong, and
the reason is not about transports at all: **nothing in the projection stream
carries the host's own `ViewState`.** A client's view slice moves only by
folding `Command`s, and a keypress resolved against the `App` moves the
*`App`'s* slice — its mode, its cursor, its minibuffer — with no `Command`
produced that any client could fold. `ClientSession::project` emits document
projections, a grid diff and a basis; none of the three is a mode change.

So a half-wired `Key` arm does not under-deliver, it **diverges**: the host
would believe it is in `AppMode::Editing` while the client draws Normal, and
`ClientSession::viewport()` reads the *mirror's* cursor, so the grid would not
even scroll to follow.

improvise-bpq2 answered it, and **not** by deriving `Command`s from a
`ViewState` delta. The user's ruling is that a keymap is UI and the server must
not know it, so resolution moves to the *client*: a key the client can answer
arrives here already resolved, as a `ClientFrame::Commands` batch it has folded
itself, and this arm's emptiness stops being a gap and becomes the statement.
What remains upstream is the set of bindings a client still cannot compose —
prefix keys, anything naming a row's category or a group, anything touching
`Storage` — and those are closed by enriching the projection, one at a time,
not by wiring this arm up. The list lives in `keys.rs` (see below).

### Persistence is the worker's, whether or not anybody asks

There is no `ModelCommand::Save`, and that is not an oversight: `effect::Save`
is `EffectKind::ViewOnly` (it writes the document to *this host's* file path
and clears *this host's* dirty flag; it mutates no document state), so the
derivation that produces `ModelCommand` from the model effects cannot reach it
(`decision(improvise-cqi, command_set_derived_not_invented)`). Saving was
therefore unreachable from any client on any transport until improvise-368h
added `ClientFrame::Save` — a *request to the host*, framed rather than
commanded precisely because it mutates nothing shared.

**A browser still never sends it**, and that is the right answer rather than a
gap: there is no file the user owns and no Ctrl-S semantics to honour, so
asking would be the wrong question — the document *is* the OPFS entry, and a
page that loses an edit because nobody pressed a key has simply lost it. So the
worker persists on its own behalf: `Worker::persist` runs the one existing
`Save` effect through `App::apply_effects`, so effects remain the only mutation
site and the save path is not duplicated. A worker with no `file_path` silently
does nothing, which is what `Save` already does; a failed save leaves its
message in the status and aborts the batch.

**The trigger is an observation of the document, not a table of command
variants.** It used to be the latter, and the reason was honest: a wire
`ModelCommand::SetCell` reached `effect::SetCell`, which did not raise
`model_state.dirty` — that was a separate `MarkDirty` effect only the key path
composed — so "save if dirty" was a trigger that never fired (improvise-gv9r).
With dirtiness raised by `App::apply_effects` off the same `EffectKind` the
`Dirty` is, the flag *is* the observation, and asking it is better than
enumerating variants for the same reason the cursor reset watches `cursor_epoch`
rather than matching on `CycleAxis`, and the rebase watches
`document_generation` rather than matching on `LoadModel`: a table of variants
is a second description of which commands change the document, free to disagree
with the first. It is also strictly tighter — a refused batch changes nothing,
so it raises nothing, and the "was anything rejected" special case that used to
guard this went with it.

One cost remains and is stated rather than hidden: `Storage` is whole-document
by construction, so OPFS rewrites the file per accepted model command —
correct, not yet cheap. And a failed write leaves the flag raised, so the next
frame retries: right for a transient failure, and harmless for a permanent one
(a worker with no `file_path`) because the only thing a failed `Save` produces
is a status line, and nothing in a worker renders one.

The one frame `on_frame` does **not** auto-persist after is `ClientFrame::Save`:
a client that asked to save has already been answered, and a failed write leaves
`dirty` set — so retrying would repeat the I/O and, on a transient failure, tell
the client `SaveFailed` about a document that is now on disk. The automatic
write exists for edits *nobody asked* to save.

So this is the browser's own answer, and `ClientFrame::Save` is the other one —
for a socket client editing a TUI-hosted document, where there *is* a file
somebody owns.

### Ordering

`Worker::on_frame` returns the frames for one inbound frame, in order, and a
`ServerFrame::Commands` batch must reach the client **whole and in sequence**:
a `GridUpdate::Cells` that overtakes the labels framing it is dropped, not
replayed. That is a property of the fold, so a `postMessage` transport owes it
exactly as a socket does — never coalesce two batches, never split one.

### Malformed frames are loud

`Worker::on_message` returns `Err` on a frame that will not parse rather than
swallowing it. A worker that silently ignores a malformed frame leaves the
client waiting forever for an answer that is never coming.

`Worker` is not `Debug`, for the reason `Host` is not: it owns the whole
document.

---

## `improvise-ws-server` — the socket host

Three layers, and the split is the point — the frames are a fourth thing this
crate speaks but does not own:

- **the frames are not here.** `ClientFrame`/`ServerFrame` are
  `improvise-protocol`'s, because the same envelopes frame the worker
  deployment and a browser bundle cannot depend on this crate (improvise-xks6).
  This crate speaks them and owns none of them.
- **`host`** — the session loop: one authoritative `App`, one `Sessions`, and a
  `handle(id, msg) -> Vec<Addressed>` that is where every decision is made and
  every test is pointed. Not here either, and for the same reason the frames are
  not: the browser worker hosts the same loop over a `MessagePort` and cannot
  depend on tokio. `src/host.rs` is a re-export of
  `improvise_command::ui::host`, kept rather than deleted so
  `improvise_ws_server::host::Host` still resolves. `Host` knowing nothing about
  sockets was always true — the file used to say "a websocket is awkward to
  test, so nothing that decides anything lives in one" — and improvise-9x6 made
  it load-bearing. A second copy is a mirror, and a mirror drifts; that is the
  same answer improvise-xks6 gave for the frames one ticket earlier.
- **`registry`** — the documents: a map of `Host`s keyed by document id, the
  lifecycle rules, and the one place a per-document `SessionId` becomes a
  server-wide address. Socket-free. It stayed on the server because a worker
  hosts exactly one document.
- **`transport`** — tokio and tungstenite: framing, JSON, the handshake rule,
  and reading the requested document off the URL path. No decisions about what
  a client is told.

Native-only: tokio's networking does not build for `wasm32-unknown-unknown`,
and it should not.

### The registry's grain

Sessions editing *independent* documents are trivially non-conflicting, which
is why ADR 001's basis tiers — `document_generation`, `layout_version`,
`record_versions` — live on the `App` and scope per document **for free**. A
registry that let two documents share any of that would throw the free scoping
away, so the registry owns no state of its own beyond the map: there is nothing
here for two documents to share.

**A `SessionId` is not unique server-wide.** `Sessions` mints ids from a counter
it owns, so every document's first client is `SessionId(0)`. The server-wide
address is therefore the **pair**, `ClientId`, and `Registry::route` is the one
place a `Host`'s `SessionId` acquires the document it came out of. That is the
whole cross-document isolation argument, and it is structural rather than
checked: a `Host` can only name its own sessions, and the only tag `route` can
apply is the document whose `Host` produced the frames. In particular an
`Applied::Rebased` fans a fresh `Snapshot` out to every client of *that*
document and cannot reach further, because the ids it returns are meaningless
anywhere else.

### Lifecycle

**Open** happens at startup, from the documents the host names. A client cannot
ask the server to open a path it was not told about, and that is not an
oversight: deciding which paths a client may open is *access control*, which
improvise-j31x puts explicitly out of scope. Serving only what was named leaves
the question unanswered rather than answering it badly. A stem collision is
fatal rather than resolved: two files with the same stem are two documents a
client could not tell apart, and silently serving one is worse than saying so.

**Close does not happen when the last session leaves.** A document stays
resident for the life of the server. Evicting it was considered and rejected
twice over:

- Unsaved edits would vanish. The server has no autosave obligation, so a
  reconnect would silently roll the document back to what is on disk.
- It would rewind the basis tiers. `document_generation` and `record_versions`
  start at zero in a fresh `App`, so a client that reconnected holding a *stale*
  basis would have it accepted as current — a correctness hole in the exact
  mechanism ADR 001 is building, paid for with a memory saving nobody asked for.

An idle document therefore costs its `App`, and `Registry::is_idle` exists so a
future eviction policy has somewhere to ask the question from — nothing calls it
to evict, because nothing evicts.

### The transport's two rules

**A client names its document in the URL** (improvise-j31x):
`ws://host:port/quarterly` asks for `quarterly`, and `ws://host:port/` asks for
whichever one there is when there is exactly one. The path was chosen over a new
`ClientFrame` variant for two reasons: a document is a *resource*, which is what
a URL path is for, and `ClientFrame` stays exactly as it was, so every client
written against the single-document server keeps working unchanged. Which
document a name resolves to is `Registry::resolve`'s decision, not the
transport's; all that happens there is `path -> Option<&str>`. It is not a path
walker — everything after the leading slashes is the name.

The other rule is that **the first frame of a connection reports the extent**,
because the bootstrap `Snapshot` cannot be built without it.

A refused connection is closed with a websocket close frame carrying the reason.
No `ServerFrame::Error` was added: the frame vocabulary is derived from the
mutations that need it, and "this connection is not happening" is a fact about
the connection, which the protocol already has a frame for.

A `Registry` is not `Send` (a `Host`'s `App` owns a `Box<dyn Storage>`), and
must not be shared anyway: it holds the authoritative documents, and one owner
is the point. So it sits in a single task on a `LocalSet` and every connection
talks to it over a channel. That is also what serializes the clients — commands
arrive in one order, and the projections each client gets are produced in that
order, which matters because a `GridUpdate::Cells` that overtakes its labels is
dropped. One task for the whole registry rather than one per document is a
*throughput* choice and nothing more: the runtime is single-threaded either way,
and no state is shared between documents by sharing a queue.

**No auth, no TLS, localhost only.** A deliberate MVP limit (improvise-q08), not
an oversight: anyone who can open a socket to this port can rewrite every
document it serves, so it binds a loopback address by default and `serve` says
so. `--bind` exists so the limit is a choice you can see rather than one buried
in the code. ADR 001 owns what multi-writer safety becomes.

### Origin

`src/origin.rs` is the handshake's one access control, and the reason it cannot
be delegated: **a websocket handshake is exempt from the same-origin policy**.
There is no CORS preflight, so any page a user visits can open a socket to the
endpoint *from that user's own address*, and read and edit every document the
server holds — cross-site websocket hijacking (improvise-kc7n). An address
check cannot see it, because the request genuinely comes from an allowed
address; an IP allow-list, a private network and loopback binding all answer
"may this **client** reach the endpoint", and the attacker's answer is honestly
yes. Only `Origin` says what is **driving** the client, which is
`doctrine(who_may_connect_is_two_questions)`. **Loopback is the least protected
case, not the safest**: there is no proxy in front of it to check anything, and
loopback is not a boundary a browser enforces, so while the server runs any page
open in that browser can reach `ws://127.0.0.1:8787`.

**The default allows loopback pages and nothing else**, because a default that
fails open is worth nothing and a default that breaks the shipped recipe is a
different bug. The two shipped paths were the constraint. The local pairing
recipe serves the page with `python3 -m http.server 8080 --directory web`, so
its origin is `http://localhost:8080` while the socket is on another loopback
port — loopback is therefore the *default*, not a flag, or the one case worth
optimising for would need configuring. The deployed Pages page sends
`https://fiddlerwoaroof.github.io` and can only reach the server through a TLS
proxy (`improvise-ws-server` speaks plain `ws://`), which is already a
multi-step setup with an Origin allow-list in it; one `--allow-origin` is
therefore an honest cost there and the project's own deployment origin is not
hard-coded as a default. `OriginPolicy::permits` is the whole decision, and the
transport's `admitted` is the only caller.

**An absent `Origin` is allowed, decided rather than defaulted.** Every browser
sends one on a handshake, unconditionally — unlike `fetch`, there is no
no-`Origin` mode to fall into — so an absent one is a non-browser client: curl,
a native client, this crate's own `tests/socket.rs`. Refusing those would break
the repo's own tests and every scripted client while buying nothing, since an
attacker who can run a non-browser client already has direct network access,
which is the question `--bind` and a proxy answer. The cost is stated rather
than hidden: **a proxy that strips `Origin` turns this check off**, so the
proxy's own Origin allow-list stays in the recipe as defence in depth.

Two shapes state no origin a policy can answer for, and both are refused rather
than read as absent: an `Origin` that is not valid text, and **two `Origin`
headers**. `HeaderMap::get` is first-wins, and the deployment this README
recommends puts a proxy in front of the socket — one that appends rather than
sets the header leaves two, and answering from the first lets the second be
anything at all. No browser sends two.

**What this does not buy is authentication.** It answers "what page is driving
this client", never "who is connecting"; anything that is not a browser page
sends no `Origin` and is unaffected by any of it. improvise-kpnr is still open
and is a different question.

---

## `improvise-protocol/src/resolve.rs` — one keypress → `Command`s

### It is not the browser's module any more (improvise-0yq8)

`resolve`/`step` and `ClientState` were `improvise-wasm-client`'s, and nothing
about either was a browser fact: both depend on `ViewState`, the protocol
widget caches and the announced window, every one of which is already a
protocol type. Being in the wasm crate meant a second front end could reach
them only by depending on a `cdylib` built for a browser, so the move cost no
new dependency and bought the rule design-principles §5 states — **a new front
end needs exactly two things**: its own event → `KeyEvent` mapping, and
painting. `BrowserKey` is the first of those two and is the only thing left in
`improvise-wasm-client/src/keys.rs`; `ClientState` is now
`improvise_protocol::client`, and `typed_cell_command` went with `step`, which
calls it.

What did **not** become trivial is `tests/lockstep.rs`. It drives
`improvise_command::ui::reduce::reduce_key` — the host's `Cmd`/`Effect` path —
against `resolve`, and those are still two derivations of the same answer, in
two crates, one of which cannot be reached from a browser. Moving `resolve`
changed which crate it is spelled in, not that it is a mirror, so
`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)`
does not fire and the test stays.

### The keymap is not this module's

It used to be. improvise-bpq2 gave this file a hand-written classifier that
decided, per key and per mode, what a keypress meant — a second spelling of
`default_keymaps()`, kept in step by a lockstep test over a hand-written list.
The user's ruling is that the web front end uses the **same** keymap as the TUI,
not a mirror of it, and
`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)` says
why: two definitions held honest by a compatibility test are one definition
waiting to be made.

So the table moved to `improvise_protocol::keymap` and this module reads it.
Which key means what is now correct **by construction**: a rebinding in
`default_keymaps()` reaches the browser with no change here, and a new binding
for a command this module can already execute simply works.

### What is left here, and why it is the better boundary

A lookup yields a command **name and arguments**. Turning that into something
that runs needs a `CmdContext` — a `Model`, a `GridLayout`, an `App` — which
this client does not have. So *key → intent* is shared and *intent → `Command`*
is ours, for the subset we can compose. `step` is that subset, and it is the
list that replaces the old list of keys. That is a list of names rather than of
keys, which is strictly better in two ways: it is enumerable, and it does not go
stale when a binding moves.

What this client can execute, and what each one needed:

- **Text entry** (`append-char`, `pop-char`, `clear-buffer`). The buffer is the
  binding's own argument now — no longer read off `AppMode::minibuffer()` and
  hoped to agree with what the host's keymap named. `TextEntry::new` derives one
  from the other, so the two were equal; they are now the *same string*. The
  pair `read_buffer`/`write_buffer` mirrors `cmd::core`'s, which are the only
  code on the host that knows a runtime-named buffer may not be an entry in
  `buffers` at all (improvise-gdl): the name `"search"` is
  `ViewState::search_query`. The shared keymap binds `append-char ["search"]` in
  `SearchMode`, so this client needs the same pair — and having it is what lets
  search typing resolve here for the first time.
- **Mode entry** (`enter-mode`), decoded through `AppMode::from_name`, which
  moved next to `AppMode` for the same reason the keymap did.
- **Grid, panel and tile cursors**, which needed `GridExtent` and the window
  this client announced.
- **Entering an edit** (`edit-or-drill`), which needed ADR 002: the branch
  `EditOrDrill` takes on "is this cell aggregated" is exactly the `CellId`
  variant the server already projects.
- **Committing one** (`commit-cell-edit`, `commit-and-advance-right`), which
  needed the record surrogate — again ADR 002 — and nothing else for the
  plain-grid case.

### The second tranche, and the three it could not reach (improvise-e24v)

Fifteen more command names resolve here: `cycle-panel-focus`; the category
panel's `cycle-axis-at-cursor`, `open-item-add-at-cursor`, `filter-to-item`,
`toggle-cat-expand` and `delete-category-at-cursor`; the view panel's
`switch-view-at-cursor`, `create-and-switch-view` and `delete-view-at-cursor`;
tile select's `set-axis-for-tile` and `cycle-axis-for-tile`; the help overlay's
`help-page-next`/`help-page-prev`; `commit-buffer` in all three of its kinds;
and the two cell writes `clear-cell` and `paste`.

Each is `Cmd`-for-`Cmd`, including the parts that look like noise: the status
sentence a command writes beside its mutation is *part of the answer*, and a
client that emitted the mutation alone would diverge on the status line and
nowhere else — which is precisely the sort of divergence nobody notices. Two
orderings are load-bearing and copied rather than rationalised.
`CommitBuffer` writes its optimistic status **first** so the add's own failure
message wins (improvise-6t3), and `TileAxisOp` writes the axis **first** and
the status after.

**Three of the tranche's own list turned out to need enrichment after all**
(improvise-4bwa), and the reasons are worth more than the code:

- `toggle-prune-empty` words its status off `View::prune_empty`, which is in
  the `Workbook`, in no cache, and not in `ViewState`.
- `delete-formula-at-cursor` needs `(target, target_category)`; `FormulaCache`
  carries the formula's raw *text*, and recovering the two from it means
  reimplementing `parse_formula` on the client.
- `view-back` emits `ApplyAndClearDrill` before `ViewBack`, and that effect is
  deliberately off the wire (gotcha 38): its operand is somebody's uncommitted
  drill edits. This one is not an enrichment at all — it is blocked on ADR 001
  layer 2's addressed form, and mirroring it by rebuilding the effect on the
  client would re-open exactly what improvise-3x3u closed.

**The lockstep harness these are tested through** (`both_ends` in
`tests/lockstep.rs`) is worth two sentences, because two of its choices were
wrong first. It compares the document as **`.improv` text**, not as `Debug`:
`Debug` prints the symbol table's `HashMap`s, so two byte-identical documents
print differently per process (gotcha 18), and the first version failed on
nothing but hash order. And it applies the client's composed commands to a
**second authoritative host** rather than eyeballing them, restating the basis
against that host with `caught_up` — the claim was published by the first host,
and the basis is the only thing that legitimately differs between two hosts
holding the same document.

**How the client claims, and why there is nothing here to keep in step.** A
client must answer the same question the server does — which records does this
`ModelCommand` write — to build its `Basis`. The first version of this answered
it *again*, in two helpers (`structural`, claiming nothing, and `record_write`,
claiming the cell's own record), and the adversarial review of PR #50 showed
that nothing tested them: `lockstep.rs`'s `apply_on` restates the basis against
the twin with `caught_up`, which re-derives it with the **server's**
`records_named` and throws the client's claim away before anything is compared.
Emptying `record_write`'s claim left the workspace green.

The fix is not a test. `records_named` is pure — `&ModelCommand -> Vec<RecordHash>`,
no `App` — and `RecordHash` is an `improvise-core` type the protocol already
re-exports, so it **moved down into `improvise-protocol`** beside the enum it
reads, and `improvise-command` re-exports it. `resolve.rs` has one `model()`
that claims `records_named(&cmd)`. The client's claim is now right by
construction, and the guard is the one that was always there: `records_named`
is an exhaustive match, so a new record-writing variant fails the build.

What the old shape would have cost is worth recording, because it is not a
compile error: `structural` accepted **any** variant and claimed `[]`, so a new
record-writing command would have compiled clean on the client with the wrong
answer, and the symptom is `Rejection::RecordUnstated` — the browser binding
becomes a dead key, refused, with nothing failing.

**`both_ends` compares every cursor**, not only the document, the mode and the
status. Three of the resolvers' decisions *are* a cursor move and nothing else —
`delete_view_at_cursor`'s step-back is the sharpest — so a comparison that
stopped at the document would agree while the panel cursor diverged.

### Mirror the `Cmd` line for line, *including where siblings disagree*

`doctrine(mirror_the_cmd_line_for_line)`. `Move` emits `SetSelected` even when
it clamps and `MovePanelCursor` returns nothing when the cursor does not move;
`MoveTileCursor` emits regardless, like `Move`. **Tidying that into one rule
would make this a second derivation**, which is exactly what `tests/lockstep.rs`
exists to rule out. Which widgets say nothing about a move that goes nowhere is
not a rule of this module — it is whatever the `Cmd` being mirrored does, and
the three differ.

Two more places the mirroring is literal. `panel_cursor` clamps a cursor past
the end *before* applying the delta, and derives the category panel's length
from `flatten_cat_tree` — which depends on `expanded_cats`, per-client, which is
why it is derived rather than sent and why the derivation lives in
`improvise-protocol` where both ends reach it (improvise-edp). And `commit`
mirrors `change_mode(exit_mode_for(edit_mode))`, then `EnterAdvance` — which
**aborts the rest of the batch** at the bottom-right corner, so the re-entry is
skipped and the user lands in normal mode — then `EnterEditAtCursor`, which
seeds the `edit` buffer from the cell it landed on. The trailing `clear-buffer`
the keymap welds to every commit then empties that buffer again. That ordering
looks wrong and is exactly what the TUI does; mirroring it is the point.

`GridMove` is a hand-spelled mirror of `navigation.rs`'s `MoveKind`, because the
command crate is not on a browser's dependency list and never will be. The
*keys* that reach it are no longer spelled twice. `scrolled_to` mirrors
`navigation.rs::viewport_effects` over the window this client announced.

### The three outcomes

`KeyOutcome::Local(Vec<Command>)` — resolved here; the caller folds them locally
*and* sends them, as one frame and in this order (the asker's own fold is its
own; the host forwards only to the others). A `Vec` because one keypress is one
binding but several `Effect`s: a cursor move is `SetSelected` plus whichever
scroll offsets follow, and a `Binding::Sequence` is several commands to begin
with. **Never empty** — a keypress this client can answer with nothing is
`Ignored`, and `KeyOutcome::of` is the one constructor so that contract is a
property of the type's users rather than a rule each resolver remembers.

`Upstream(KeyEvent)` — bound to a command this client cannot compose.
`Ignored` — nothing bound, or what is bound resolves to no change. **Nothing
bound is not "the host might know better"**: the host looks the key up in this
same table. That is new; it used to go upstream into an empty vec.

A `Binding::Sequence` is **all or nothing**: half of `Esc` — clearing the buffer
without leaving the mode — is worse than none of it. Each step is resolved
against the *unchanged* state, because the host resolves every step against one
immutable `CmdContext`; the commands are then folded in order.

### What still needs the host

`the_command_names_that_still_need_the_host_are_these` names them, and
`Host::handle` answers a `Key` frame with an empty vec, so each one is a binding
that **does nothing at all** in a browser
(`doctrine(an_upstream_key_is_a_black_hole)`).

**An upstream key that the app also ADVERTISES is a different severity, and it
had its own class of defect** (improvise-rmvx). A key nobody offers doing
nothing is a gap; a key named in the mode's own hint line doing nothing reads as
*broken*. The two are crossed by `every_advertised_command_answers_in_its_own
_mode`, which asks `help::advertised` — a table of command **names**, so the
cross-reference is exact and nothing tokenises the rendered string — against
`resolve` in every mode, over a fixture furnished enough that an `Upstream` can
only mean an unhandled name. The old guard asked only about `Normal`, which is
why `add-record-row` and `delete-formula-at-cursor` were outside its reach
entirely; `view-back` in RecordsNormal was a seventh instance nobody had
noticed.

**`Ignored` is a black hole too**, and the guard watches both variants.
`KeyOutcome::of` turning an empty `Local` into an honest `Ignored` is good
design and also a **laundering** step — every wrong answer that composes nothing
arrives wearing the variant that is usually innocent. A guard's discriminator
must therefore be *state* (did anything the user can see change) and never the
shape of the return value. Watching only `Upstream` let three of the six
declines go completely silent with 1397 Rust, 19 harness and 26 node tests
green, because no browser spec presses those three keys (PR #71 review).

`nothing_declined_is_also_composed` closes the other direction: `step` reaches
`declined` first, so an arm in `composed` for a declined name is **unreachable**,
and the browser would go on saying "not available yet" over working code. It is
derived from `declined` itself over every bound `(name, args)` in every mode, so
a seventh decline is covered without editing it. `enter-mode` was a real overlap
on its first run; rather than move the check, **both halves hold the rule** —
`composed` returns `None` for a stranded mode — so removing one fails towards
`Upstream` rather than towards a dead room.

`declined` is the answer, and it is one list rather than an arm apiece: six
command names, each with **its own true subject**. Wording matters here more
than it looks — the `>` fix was nearly routed through a message about
aggregates, which would have been true-sounding about the wrong thing and worse
than silence, because a reader who is told something plausible stops looking.
`step` checks `declined` first, so `composed` has no arm for any of them and the
two lists cannot overlap.

A declining step takes its **whole sequence** with it: `o` in RecordsNormal is
`[add-record-row, enter-edit-at-cursor]`, and the second step's premise is false
once the first has declined. Returning only the decline, rather than the decline
plus whatever composed before it, is what keeps `Binding::Sequence`'s "every
step or none of them" true — an abort is not a rollback (gotcha 21), and this
client has no way to undo a `Command` it has already put on the wire.

The classes that remain:

- **Prefix keys.** `Command::SetTransientPrefix` exists and this client could
  arm one; what it cannot do is what the *second* key then runs, which is
  `yank`, `wq` and `toggle-col-group-under-cursor`. Arming a prefix whose
  payload is unreachable is worse than not arming it.
- **Anything naming a row, column, item or group** — `hide-selected-row-item`,
  `toggle-group-under-cursor`, `delete-formula-at-cursor`. The first two are **no
  longer blocked** and are simply not built here yet (improvise-e24v): since
  improvise-anr9 `GridCache` carries an `AxisFrame` per axis, so `cats[0]` +
  `label(idx, 0)` is exactly what `HideSelectedRowItem` reads off `GridLayout`,
  and `group_of(idx)` is exactly `ToggleGroup`'s `(category, group)`. The one
  thing to get right is that the frame is **windowed** and the cursor is
  absolute, so a client must reconcile the scroll offset first, as `render.rs`
  does for the selection. `delete-formula-at-cursor` stays: it names a position
  in the formula panel, not in the grid.
- **Anything touching storage or an arbitrary command line.** The host's by
  definition.
- **The records-mode commit** — *except the two creating columns*. A wide-records
  row is one coordinate tuple and editing a category column moves every measure
  cell in it; `commit_plain_records_edit` needs `GridLayout::record_col` to know
  which. A `CellId::Create` does not: it commits the typed NAME as one existing
  `ModelCommand` (`AddItem` into `_Measure`, or `AddCategory`), reads no
  `record_col` and writes no cell, so improvise-gcvb narrowed the early return
  rather than removing it. The `+cat` half **must not advance**: an inserted
  column renumbers every index to its right and the advance was computed from
  the pre-commit layout (gotcha 23), so `CommitAndAdvance` returns straight
  after it and a client that advanced anyway would land one column off,
  silently. `AddCategory` can also be refused by the 12-category limit and sets
  `abort_effects`; the mode change therefore goes **first**, where an abort
  cannot reach it (gotcha 21).
- **Drill.** Session-local residue, and the one case that does **not** go
  upstream silently — see below.

#### The three improvise-e24v listed as free, and what each really costs

improvise-4bwa. Each of the three is short exactly one fact, and the three
answers are different in kind — which is the point, because "needs enrichment"
read as one bucket is what let them be listed as free.

**`view-back` is declined, permanently as things stand, and it is not an
enrichment at all.** `ViewNavigate { forward: false }` emits
`ApplyAndClearDrill` *before* `ViewBack`, and `ApplyAndClearDrill` is
deliberately off the wire: its operand is `drill_state.pending_edits`, session-
local residue belonging to whoever staged it, and a `Command::Model` is routed
with no `SessionId` (gotcha 38, improvise-3x3u). A client that mirrored
`view-back` by reconstructing the drill commit would commit **somebody else's
uncommitted typing** — which is the exact thing 3x3u removed, and the reason it
must not be "mirrored line for line" here despite that being the rule everywhere
else in this module. So it stays upstream until ADR 001 layer 2 gives the
addressed form. Note the sharper reading: `mirror_the_cmd_line_for_line` is a
rule about *resolvers*, and a `Cmd` whose first effect is unaddressable is not a
`Cmd` a thin client may mirror at all. The doctrine does not conflict with this;
it simply does not reach it.

**`toggle-prune-empty` needs one bool on a per-view projection.**
`TogglePruneEmpty::execute` reads `ctx.view.prune_empty` only to word its status
— "Showing all rows/columns" against "Hiding empty rows/columns". The
`ModelCommand` itself is composable today; it is the sentence beside it that a
client cannot write, so mirroring it would put the two ends out of step on the
status line in exactly half of all presses. `View::prune_empty` is on the `View`
inside the `Workbook`, in no widget cache and not in `ViewState`.

**`delete-formula-at-cursor` needs an element-type change, and the shape of the
change is the whole decision.** The effect is `RemoveFormula { target,
target_category }`; `FormulaCache` carries the formula's *raw text*
(`"Profit = Revenue - Cost"`), from which neither field is recoverable without
reimplementing `parse_formula` in the client. Adding a `targets` vector
**beside** `formulas` would be two answers to "what is in the formula panel",
which is improvise-edp's disease and the mistake improvise-anr9 had to pay a
type change to undo. So the element type grows the fields; it does not acquire a
parallel array.

Both of those are ordinary projection additions and both live in
`improvise-protocol` (`render.rs`, `resolve.rs`) and `improvise-command`
(`ui/project.rs`) — not in this crate, which is why this section records the
design and the work lands with whoever owns those files. Whatever lands, the
named list `the_command_names_that_still_need_the_host_are_these` shrinks by
exactly what closed.

#### What a client that could parse would cost: measured

improvise-cl7y asks a client to stop claiming "Formula added" for text only the
host can parse. Its own note says the two routes are a dependency decision or a
status addressed back to the composer, and leaves the cost of the first
unmeasured. It is measured now, by adding `improvise-formula` to this crate,
calling `parse_formula` from an exported method so nothing is stripped, and
building the bundle both ways:

| | `improvise_wasm_client_bg.wasm` | gzipped |
|---|---|---|
| today | 503,854 | 191,613 |
| with `improvise-formula` | 634,515 | 238,973 |
| cost | **+130,661 (+25.9%)** | **+47,360 (+24.7%)** |

The raw column is exact and byte-verified; the gzipped one is a `gzip -9`
**proxy** and not what a visitor downloads, since GitHub Pages serves brotli.
The conclusion is unaffected — the raw figure alone carries it — but do not
quote "+47,360 gzipped" as a transfer size.

A quarter of the download, for a status string. That is the evidence for the
other route — the host addressing a status back to the composer, the way
`ClientFrame::Save`'s rejection already does — and it costs no dependency.

Note what the measurement does **not** settle, because it is the more
interesting half: a client that could parse would be a **second derivation** of
whether a formula is well formed, agreeing with the host only for as long as
both call the same function. The status line is not the only thing that wants
it, so the number above is a floor rather than the price of a feature.

### `stranding`: a prompt this client can enter but cannot commit

`enter-mode` is trivially resolvable here, and that is the trap: the user
reaches the prompt, types a path, presses Enter — and the commit resolves to
nothing, because `Host::handle` answers a `Key` frame with an empty vec. **A
dead room is worse than a locked door**: the door costs one inert key, the room
swallows work the user believes was done.

`SavePrompt` and `ExportPrompt` are host-only **by construction**, not by
omission: `effect::Save`, `SaveAs` and `ExportCsv` are all
`EffectKind::ViewOnly`, so the derivation that builds `ModelCommand` from the
model effects cannot name them. Closing this is not a resolver to add; it is a
wire operation that does not exist. In a browser it is also close to
meaningless: the worker persists on its own behalf whenever the document is
dirty, so the document *is* the OPFS entry and there is no Ctrl-S semantics to
offer.

It returns the **subject** rather than a bool (improvise-rmvx): the refusal is
right and the silence was not, so `:` now says the command line is unavailable
instead of doing nothing at all. The predicate is named as a property rather
than matched at the call site (`doctrine(two_questions_one_enum)`): "can this
client finish what entering the mode starts" is not the same question as "is
this a text-entry mode", and a third such prompt belongs here. Found by Copilot on PR #27 against the newly
added `SavePrompt`; `ExportPrompt` had the identical trap already and was fixed
with it — fixing one arm of a rule with two is the shape
`doctrine(one_arm_of_a_symmetric_rule_is_evidence_about_that_arm_only)` names.

**`CommandMode` is dead by omission, and listed anyway.** Copilot found it on
the next round, and it was a false negative in the predicate's *first* version —
which mattered more than its severity, because
`a_prompt_this_client_cannot_commit_is_never_entered` used `:` as the negative
arm that stops "refuse every mode" from passing. An arm asserting something
false does not merely fail to guard; it **certifies the behaviour it was added
to rule out.** `:` resolves locally into `CommandMode` and typing appends
locally, but Enter is `[execute-command, clear-buffer]` and `step` has no
`execute-command` arm, so `resolve` returns `Upstream` and the frame is dropped.
Every ex-command behind `:` is inert for the same reason, so refusing the mode
removes **no working surface**, only a room that swallows typing.

The distinction from the two prompts is worth keeping visible, because it says
who removes that line. They are dead **by construction**: no `ModelCommand` can
ever name `Save`/`SaveAs`/`ExportCsv`, so no resolver would help. `CommandMode`
is dead **by omission**: `execute-command` is implementable client-side and
improvise-e24v is scheduled to do it. Listing it is therefore also the named gap
gotcha 47 asks for — closing e24v has to come to that line and delete it.

### Per-arm notes that would otherwise be re-derived wrongly

- **`enter-tile-select`.** `EnterTileSelect` gates on
  `model.category_names().len() > 0`, which counts the **virtual** categories
  too — `_Index`, `_Dim` and `_Measure` always exist — so the gate is
  invariantly true and there is nothing here to mirror it against. Re-deriving
  it from `CategoryTreeCache`, which carries the panel's rows rather than
  `category_names()`, would be a second derivation that disagrees on exactly the
  empty document.
- **`transpose`.** It used to be emitted alongside a `mark_dirty` this client
  had no wire variant for; since improvise-gv9r dirtiness is raised by the host
  off the effect's own `EffectKind`, so there is nothing to mirror and nothing
  missing. The cursor reset is not this client's to send either: the effect calls
  `App::reset_cursor`, and `adopt_cursor_epoch` broadcasts `Command::ResetCursor`
  to every client on a view the batch reshaped (improvise-nynk) — so sending one
  here would be a second, differently-addressed copy.
- **`toggle-panel-and-focus`.** The interactive arm of `TogglePanelAndFocus`:
  open-and-focus unless this panel is already both, in which case close and
  unfocus. The decision is made in `registry.rs` on the host and here on the
  client, from the same two `ViewState` facts.
- **`edit-or-drill`.** The host asks whether a regular category sits on
  `Axis::None` and whether the cursor is on a synthetic records cell. This
  client asks neither: the server already published the answer as the `CellId`
  variant, and `doctrine(a_demo_is_checked_against_the_type_not_the_axes)` says
  to read the type rather than re-derive the rule. It reads **`edit_text`, never
  `text`**: the drawn text has been through `format_f64`, so a stored 1200 reads
  `"1,200"` under a comma-grouping format and committing it would store the
  *string* `"1,200"`. `tests/lockstep.rs` found exactly that.
- **Drill does not go upstream — by *either* door.** Every `Upstream` is a black
  hole, and a key that visibly does nothing is the worst of the three answers.
  Drill is session-local residue and reaching it needs a `CellKey` this client
  cannot build, so the honest answer is to say so: a `SetStatus`, which is a
  view-half command — `Sessions::apply` folds it into the asker's own mirror and
  broadcasts nothing, so it reaches this user and no one else. This argument
  arrived as `edit_or_drill`'s, i.e. about `i`/`a` on an aggregate, and for a
  while it read as doctrine for `>` while `>` was still silent. Both doors now
  refuse, and **they refuse with different sentences on purpose**: "this key is
  not built yet" and "this cell is not yours to write" are different facts.
- **The commit's write is not spelled again here.** It is
  `Client::typed_cell`'s, the same one click-to-edit uses, so a browser keyboard
  commit and a browser mouse commit cannot disagree about what `""` or `"42"`
  means.
- **`cell_at_cursor` returns `None` when the selection is scrolled out of the
  window** — not a state the cursor keys can produce, since they scroll to
  follow, but one a projection race can.
- **`grid_move`'s two empty shapes** are kept apart from "not ours": an empty
  grid has nowhere to move to, and a client that has not announced a viewport
  has been sent no grid either. Neither is a key the host could answer better.
- **`Quit` has no keymap**, on purpose: `ModeKey::from_app_mode` is exhaustive so
  that a new mode without bindings is a compile error rather than a mode that
  swallows every key.

### Browser key → improvise key

`to_key_event` returns `None` for two cases the host must never see. A bare
modifier press (`"Shift"`, `"Control"`, …) is not a keypress. And anything held
with the platform meta key is left to the browser: `KeyModifiers` has no bit for
Meta, so forwarding `Cmd-R` would arrive as a plain `r` and type into a cell
instead of reloading the page.

Two conversions are not obvious. A terminal reports **Shift-Tab** as its own
code and the keymaps bind that code; the browser reports Tab plus a shift flag.
And **shift is deliberately dropped for character keys**: the browser already
applied it (`"A"`, not shift+`"a"`), and the shared keymap's `lookup_local`
retries character keys without modifiers for that very reason, so keeping the
flag would only make an exact binding miss.

### The pointer: `Click`, and the four rules it obeys

A click is resolved here for the same reason a keypress is — a front end paints
and wires, it does not decide (design-principles §5). `resolve::click` takes a
`Click` naming a widget and an index, and returns the `Command`s. `web/app.js`
attaches listeners and knows nothing else; there is one entry point per widget
on `Client`, which was the user's ruling on improvise-7ybz.

**Rule 1 — a click navigates to what was clicked, and activates it where
activation is something the client can express.** Every arm composes the same
shape: `focus` the panel, `SetPanelCursor` to the row, then whatever `Enter`
does on that row. The activation is resolved **against the clicked index, never
against the cursor**, because the `SetPanelCursor` ahead of it has not been
folded when the resolver runs — the trap that reads as "the cursor lands on
`Plan` and the document switches to `Default`", silent and wrong in the half a
user does not watch.

The second clause of rule 1 is what makes the formula panel look inconsistent
and is in fact the rule holding. `ModeKey::ViewPanel` binds `Enter` to
`switch-view-at-cursor`; `ModeKey::FormulaPanel` binds `a`/`n`/`o` and
`d`/`Delete` and **no `Enter` at all**. So a view-row click switches and a
formula-row click only navigates. Opening the editor on a formula click would
be an activation *invented for the pointer that the keyboard does not have* —
a second derivation of what a formula row means, which is the disease §5 names.
**The guard for this must be aimed at the keymap, not at the click**: asserting
that the click emits no `FormulaEdit` only restates the author's reading, and a
reviewer demonstrated the gap by adding `Enter -> enter-mode formula-edit` to
`FormulaPanel` and watching the whole workspace stay green while the click and
the keyboard disagreed. `the_formula_panel_has_no_enter_to_mirror_and_the_view_panel_does`
asserts the binding's absence and names `click_formula_row` as what must change
if it ever appears.

**The browser assertion that PR #60 dropped stays dropped, and improvise-qf57
is the decision rather than the oversight.** `#grid input` `toHaveCount(0)` was
argued to be redundant with the `#mode` assertion on the grounds that
`toHaveText("FORMULAS")` is exact and `AppMode::FormulaEdit` labels as
`"FORMULA"` — a coincidence of two strings differing by one character, which
would be a thin reason to leave it out. Measuring it gives a better one: it is
not redundant, it is **unreachable**. `web/app.js` creates a grid `<input>` in
exactly one place, the grid cell's own `mousedown`, gated on `cell.editable`;
no repaint creates one and no panel handler can reach it. And an invented
formula-row activation opens the **minibuffer**, not a grid editor. Injecting
`Command::ChangeMode(AppMode::formula_edit())` into `click_formula_row` and
running the spec: `#grid input` `toHaveCount(0)` **passes**, and the run fails
on the `#mode` assertion below it. Restoring it would add an assertion that
cannot fail — the same defect this file already records one line down, where an
explanatory `assert!` was removed for never being able to be the failing one.

**Rule 2 — a click is all of its intents or none of them** (improvise-hkr8).
Each `click_*` returns `Option<Vec<Command>>` and `click` does the single
`unwrap_or_default`. The case that matters is an activation the client cannot
compose: `filter_to_item_at` goes through `model`, which goes through
`BasisCache::claim`, which returns `None` until the server has published a basis
(gotcha 43). Before the fix the `None` was swallowed per-intent, so the click
kept its navigation prefix, dropped its activation, and reported success — while
the *keyboard* handled the identical `None` correctly by answering
`KeyOutcome::Upstream`. Note that `None` at the `click` level means something
different and benign: the whole click composed nothing, which is the invariant
rather than a breach of it.

**Rule 3 — a click on an *unselected* tile never mutates the document.** The
user's ruling on the tile bar. The cost accepted is two clicks for the common
action and a **deliberate divergence from the keyboard**, where `Enter` cycles
directly — record it as chosen, not as an inconsistency to tidy, or someone will
read the binding and "fix" the pointer to match. What it buys is worth more than
the keystroke: a tile cycle is a `LayoutGeometry` change, which bumps
`cursor_epoch` and therefore resets **every connected client's cursor** (gotcha
24), so a stray click used to cost every other viewer their position.

**State the rule this precisely, because the looser wordings are false.** The
implemented rule is "clicking the already-selected tile cycles it; clicking any
other selects it", where *already selected* is `mode == TileSelect` **and**
`tile_cat_idx == index`. The tempting summaries — "a click never mutates on its
own", "a **second** click cycles" — both have the same counterexample, found on
review: `T` enters `TileSelect` leaving `tile_cat_idx` at **0**, so a click on
the **leftmost** tile is a click on the already-selected one and cycles on the
*first* gesture. Two adjacent tiles therefore answer the identical first click
differently, and that is correct rather than a hole — a click does what `Enter`
does there, and `Enter` in `TileSelect` cycles the tile under the cursor. The
resolver cannot see *how* the state arose and must not try to.

The mode conjunct is still load-bearing in the other direction: without it,
`tile_cat_idx == 0` in **Normal** mode would read tile 0 as already selected, and
the very first click of a session on the leftmost tile would cycle it.

**Rule 4 — the shared helper is the idiom, and breaking it is how the two arms
drift.** `click_category_row` calls `filter_to_item_at`, `click_tile` calls
`tile_axis_at`, `click_view_row` calls `switch_view_at` — each is `foo_at(index)`
with the keyboard's `foo_at_cursor` calling `foo_at(cursor)`. `click_view_row`
originally re-spelled `SwitchView + ChangeMode(Normal)` inline, so a host-side
change to `SwitchViewAtCursor` was forced into the keyboard arm by
`tests/lockstep.rs` and left the click behind **silently**. A helper that returns
only the *name* rather than the whole activation is what invites that.

### The category panel opens at launch, in both front ends

`ViewState::default()` sets `category_panel_open: true`. The user ruled this
twice: first that the sidebar be visible at launch, then — asked whether the
terminal was collateral — that it applies to **both**. `improvise model.improv`
opening with the sidebar showing is intended, not a side effect of sharing a
default. It is the **shared** default for the reason wsaw gives: a browser-only
default would be "the page overrides the flag", which is the same defect as "the
page ignores the flag" wearing different clothes.

**A launch default disarms every test that asserted the thing it now supplies**,
and that is the expensive part. Three tests asserted `category_panel_open` was
*true* after doing something to make it true; all three then passed without the
code under them working. The worst was `set_panel_open_and_cursor`: with it
vacuous, `effect::SetPanelOpen`'s Category arm could be replaced by a **no-op**
— `C` doing nothing in the terminal — with the whole workspace green. The rule
this leaves: **assert the direction the default cannot supply.** Closing the
panel is the discriminating move; opening it proves nothing.

Note the two mappings that had to be repaired separately. `effect::SetPanelOpen`
and `reduce_view`'s `Command::SetPanelOpen` are two three-way matches in two
crates — the TUI folds one and the browser the other — so a guard on either says
nothing about the other. That is a derivation done twice (§5), pre-existing and
filed rather than fixed here.

Two tests, because either alone passes against the bug wsaw fixed: the panel is
visible with no keypress, *and* `C` still hides it. Injecting "the page ignores
the flag" fails the second; injecting "the default is false" fails the first.
A test that only asserts the sidebar is visible cannot tell the flag being
honoured from the flag being ignored.

### A black hole that leaves the keyboard in a mutating mode (improvise-koh6)

This file's list of what a client cannot compose used to read as a list of keys
that *do nothing*. `N` was on it, and it is worse than nothing. `N` is
advertised on the browser's **own** help overlay ("Quick-add a new category,
from anywhere"), it resolved `Upstream`, `Host::handle` answers a `Key` frame
with an explicit empty vec — so the DOM was byte-identical — and the user then
typed the category name into NORMAL, where `i` is bound to `edit-or-drill` and
opens the grid editor **pre-filled with the cell's current value**. Measured on
`web/demo.improv`: `N R e g i o n Enter` turned `18,400` into the text
`18400on`, with no category created. A dead key in a *modal* editor is a data
loss, not a papercut, and that distinction now belongs on the list.

**The decision it forced.** `SearchOrCategoryAdd::execute` is a conditional: a
non-empty `search_query` delegates to `SearchNavigate`, which a browser cannot
compose (it would have to search a grid it holds only a viewport of); an empty
one emits two commands this client already has. Composing the second arm and
returning `None` for the first would make **whether the host is needed** depend
on client state, which `enter_in_an_empty_view_panel_resolves_locally_rather_than_upstream`
exists to forbid.

The resolution is that the rule is about `Upstream`, not about outcomes. The
name is now answered in `composed` in **both** arms — the composable one builds
the pair, the other declines through `not_yet` with its own true subject — so
the host-needed list stays a property of the *name*. The outcome varying with
state is what `cycle-axis-at-cursor` and `paste` already do a few arms above;
what must not vary is whether an answer exists at all. `declined()` could not
express this, being keyed on the name alone, which is why the sentence moved
into `not_yet` and the arm lives in `composed`: `step` reaches `declined`
first, so the two lists still cannot overlap and
`nothing_declined_is_also_composed` still holds.

Note the arm mirrors the `Cmd` line for line, including that it uses a plain
mode change rather than `EnterMode` and so does **not** clear the minibuffer on
the way in. Tidying that would be a divergence, not a tidy.

### A cursor that does not follow a shrinking list (improvise-uej8)

`d` on the last item of an expanded category shrank the tree under a cursor
that stayed put, so the row beneath it became the **next category's header** —
and one more `d` removes a whole category, which is `LayoutGeometry` and
rewrites every view's axis map. The same `Cmd` backs the same key in the TUI;
this was never browser-specific.

The rival fix is the one `delete-view-at-cursor` and `delete-formula-at-cursor`
already use — step back whenever the cursor is above zero — and on this list it
is *worse than the bug*: the first item of a category would retreat onto that
category's own header, the same escalation one row earlier, and holding `d`
would walk backwards instead of clearing the category out. Those two panels are
right to do it because their lists are **homogeneous**; a cursor that does not
move on a one-kind list cannot escalate. That is the property to check before
copying a cursor rule between panels, not the panel's name.

**The first fix was the adjacent-only one — step back when the row below is not
a sibling — and a ChatGPT Codex review of PR #114 found it wrong for a category
with exactly ONE item.** That is the configuration in which the conditional
rule degenerates into the unconditional one: no sibling below, so it steps back
onto the category's own header, and the escalation returns. The paragraph above
had already named that shape as the reason not to copy the other panels' rule;
it just did not notice that the singleton reaches it too.

`CatTreeEntry` has two variants and `flatten_cat_tree` emits a header then its
items, so the enumeration is short. After deleting the item at `i`: if the row
below is a sibling the list closes up and the cursor stays; if the row above is
a sibling it steps back; **and if neither, the category is now empty and BOTH
neighbours are `Category` rows** — there is no safe adjacent landing at all.
That third case is structural rather than an oversight, and it is why the rule
could not be patched in place.

So the landing is the nearest **`Item`** row, preferring above, then below.
`d` there deletes an item, never a category, which is the criterion the review
asked for. The cost is real and is not hidden: the cursor may cross into
another category, and a held `d` then deletes one item per press — each with
its own status line — walking upward rather than escalating. Landing on the
emptied category's own header was rejected even though `RemoveCategory` on an
empty category is cheap (no cells, and a non-measure formula target *is* an
item, so no formulas): that is a harm-magnitude argument, and this repo prefers
making the bad state unreachable.

One residual, stated rather than guarded: if the deleted item was the only item
in the whole tree there is no `Item` row anywhere, so the cursor does not move.
Every category is empty at that point, so the escalation it leaves is the cheap
one, and no landing exists that would avoid it.

### One operation, two routes, one sentence (improvise-25bf)

`cycle-axis-at-cursor` from the category panel emitted no status where the tile
route emitted `Product → None`, so the same operation told the user only down
one of its two paths — design-principles §5 in miniature. The wording now has
one home, `viewmodel::axis_change_status`, called by both routes here and by
both in the TUI.

This also pays for improvise-z336's third finding, which is the more
interesting half: the `Filter: X = Y` status persisted after the axis had moved
to None, Row and Col, so the one piece of feedback about a filter was wrong
exactly while the user was trying to be rid of it. A user who *had* cleared it
correctly was told for the rest of the session that it was still on — a
mechanism that manufactures a bug report out of a working code path. It needed
no separate fix: a route that writes a fresh status writes over the stale one.

---

## `improvise-wasm-client` — the crate, the fold, the render model

### What the crate is, structurally

One `wasm32` artifact, reused literally in both deployments: the thin client
talking to a server over a WebSocket and the standalone app talking to a worker
that hosts the `App` over the worker's own port. Only the JS bootstrap differs —
nothing in the crate knows what the transport is.

It is a **fold and a key classifier**, and nothing else. It holds a `ViewState`
plus one widget cache each — there is no aggregate render cache — and it moves
them by folding the same `Command` stream the host folds. The fold is `state.rs`;
the key classifier is `keys.rs`.

The argument that shaped it: a keymap binding names an **intent** resolved
against a `CmdContext`; a `Command` is the **resolved mutation**. A thin client
holds no `Model` and no `GridLayout`, so it cannot do that resolution. (Since
improvise-bpq2 the *table* is shared and the resolution of the composable subset
happens here; what is left upstream is the subset `keys.rs` names.)

That is why the command crate is a **dev**-dependency and not a dependency: its
keymap could not be used here even if the bundle could afford `App`, `Workbook`,
persistence and the import wizard. What the dev-dependency buys is
`tests/lockstep.rs`, which drives the authoritative host and this client over the
same keypress and asserts the two `ViewState`s agree — the correspondence is
checked rather than asserted in prose.

**The boundary is JSON**, so the JS side needs no generated structural bindings
and the same strings go on a socket unchanged. No `std::fs` (it compiles for
wasm and fails in a browser) and no clock: nothing here calls `Instant::now()`,
so the `web-time` shim the command crate needs is not needed. Both are
structural, not disciplined — this crate depends only on `improvise-protocol`,
`serde`, `serde_json` and `wasm-bindgen`, none of which reaches a filesystem or
a clock.

**Ordering.** The JS bootstrap must deliver frames to `Client::on_frame` in
arrival order, one at a time. The grid fold is deliberately not commutative — a
`GridUpdate::Cells` that overtakes the labels framing it is dropped — so batching
or reordering silently loses cells. For the same reason a locally-resolved
command and a forwarded keypress must go out on the *same* ordered channel, in
the order `Client::on_key_event` returned them.

**Why the `wasm_bindgen` surface is a thin wrapper over a plain-Rust one.**
`wasm_bindgen` methods cannot take or return the protocol's types, so there is
one code path serialized at the edge. The split is not only ergonomic:
`JsError::new` panics off wasm, so every fallible path has to be *reachable*
without it or its failure arm cannot be tested at all.

**`Client::viewport` vs `Client::set_viewport`.** `viewport` is associated
rather than a method, because a page must send the handshake *before* it has
anything to fold. But it builds the frame and forgets it, so a client driven
only through it never learns its own window and therefore resolves no cursor key
locally — a truthful subset (those keys go upstream and the host answers
nothing) but a whole feature quietly off. `set_viewport` is the one a page should
call. The window is **not derivable** from anything the host sends: the row
frame's own length is `min(window, grid − offset)`, so it agrees with the window
except near the bottom edge, which is precisely where the scroll arithmetic
matters. `viewport` is kept because `web/app.js` already spells it and retiring
it is a co-ordinated change.

### `improvise-protocol/src/client.rs` — the two properties this dispatch owes

Both are tested in `state.rs` rather than assumed from the protocol crate's own
tests, because the dispatch between them is this crate's.

- **The stream is not reordered.** Every fold path is a `for` over arrival
  order; there is no batching, sorting or coalescing.
- **Bootstrap is a compaction, not a replay.** `ClientState::bootstrap` folds
  `Snapshot::seed` exactly as the steady stream is folded, so there is no second
  code path that could disagree with the incremental one. `GridCache::seed` puts
  the labels before the cells for the same ordering reason.

**`window` is deliberately not reset by `bootstrap`**: the browser window did
not change size because the document was replaced, and clearing it would
silently stop this client resolving its own cursor keys until the next resize.

**`last_rejection` is kept as a value, not a sentence.** A rejection is the
input to *force*, and force is a re-composition against the republished basis —
which needs to know **which** record and **which** kind. `RecordGone` cannot be
forced at all; `RecordMoved` can be forced with one keystroke. A string cannot
carry that. It is session-local: it belongs to whoever composed the refused
batch, so it is not part of the portable slice and does not survive a bootstrap.

`window` is session-local and not portable for the same family of reasons: it is
a fact about a browser window, which is why no `Command` carries it and why it is
not on `ViewState`. `(0, 0)` means this client has not announced one yet, and a
client in that state resolves no cursor key — it has been sent no grid either.

### `socket.rs` — the frames, the wording, and the one composition

**Where the frames live: not here.** `ClientFrame`/`ServerFrame` are
`improvise-protocol`'s, and the same pair frames both deployments. This crate
used to carry two envelope pairs of its own: one *mirroring*
`improvise-ws-server`, because a native crate cannot be a dependency of a wasm
bundle, and one written for the worker before the projection architecture
landed. The mirror was held honest by two tests round-tripping every variant
through the server's own types; with one definition there is nothing left for
them to compare, which is what improvise-xks6 was for.

**`rejection_message` is spelled here rather than on `Rejection`** for the
reason `AxisColor` is not a ratatui colour: the protocol carries the *fact*, and
each front end words it. The wording tracks ADR 001's force table, because "try
again" and "this cannot be forced" are the two different things a user must be
able to tell apart — a `RecordGone` has nothing to write to, and offering force
there would promise something the design refuses to deliver (**force never
creates structure**). Two wordings are load-bearing: `DocumentReplaced` must not
say "reloading", because nothing here reloads, and the two save failures are not
staleness so neither offers a retry — retrying changes nothing about the host's
storage.

**`typed` is a rule that genuinely lives in two places.** It is the same
three-way split `commit_regular_cell_value` makes — empty clears, a parseable
number is a number, anything else is text. It is duplicated here for the reason
the whole client exists: the browser has no `App` to ask, and
`ModelCommand::SetCell` takes a `CellValue`, so *somebody* on this side must
decide. Putting it in Rust rather than in the page's JavaScript is what makes it
testable and keeps one spelling per language boundary instead of two.

This is the **only** answer the *client* gives to "what does a typed string
mean" (improvise-r4lo), and it holds on every deployment including the worker: a
host's `App` sits behind the frame boundary and is handed a value, not a string,
so no host ever classifies a string that arrived *over the wire*. The careful
half: a host does classify its own **local** keystrokes, which is why this is a
duplicate of that split rather than a call to it — the command crate is
deliberately not a dependency, and reaching it would drag `App`, `Workbook` and
persistence into the browser bundle.

So `a_typed_string_is_classified_as_the_host_classifies_it` **states the
obligation without discharging it**: it asserts the three outcomes by hand
rather than comparing with the host's function, so the two could drift and stay
green. That is
`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)` still
open, one layer below what improvise-r4lo closed — the real fix is a shared
split in `improvise-protocol`, which both crates already depend on. Filed rather
than done.

`typed`'s `None` means "clear this cell", which is a *different command*, not a
different value — hence the `Option` rather than a `CellValue::Text("")`.

**`typed_cell_frame` is the composition improvise-q08 found a thin client could
not do.** It holds no `Model`, so it cannot build a `CellKey` — but it does not
need to: the id on the cell it is already drawing *is* the address, and it is
echoed back unread (ADR 002). It takes the user's **string**, not a `CellValue`,
for the reason above; there is no deployment for which a pre-classified entry
point would be right. A sibling taking a serialized `CellValue` used to sit
beside it and claim the opposite; it was deleted rather than re-documented,
because it could not express `ClearCell` and was therefore a strictly weaker
duplicate.

`None` on **four** shapes, each a different fact the client already holds: the
position is outside the grid; the position holds no settable cell (`id: None` —
padding, or a records-mode column that renames a coordinate rather than writing
one); the cell is an **aggregate**, which is the pattern failing to bind rather
than a check; or this client has not been told the document's basis and so has
nothing truthful to say about what it composed against (`BasisCache::claim`
withholds rather than invents).

**Nothing is folded there.** The new value comes back as a `GridUpdate::Cells`
from the host, which is the side that formats it; guessing it locally is how the
two ends drift apart.

`typed_cell_command` is the one composition shared by the mouse path
(`typed_cell_frame`) and the keyboard path (`keys::commit`). Writing it twice is
how a browser commit by Enter and a browser commit by click would come to
disagree about what an empty string means.

### `render.rs` — the four decisions that are silent when wrong

The DOM half of this client is JavaScript. The split is drawn to make that
acceptable rather than comfortable: **every question with a wrong answer is
answered here, in Rust, under test**, and what crosses to the page is a list of
rows of cells that each already know whether they are editable, numeric,
erroneous and selected. The page's job is `document.createElement`.

- **Editable is a fact about the id, not about the text.** Only
  `CellId::Record` may be written (ADR 002), and there are now **four** kinds,
  each naming a permission. `CellId::Aggregate` is a *drill* target and `Drill`
  is not on the wire yet, so it renders as non-editable rather than as a control
  that does nothing. `CellId::Derived { target }` is a cell a formula owns
  (improvise-ir7l): `i` on one names the formula rather than opening an editor,
  because a write there lands and is then shadowed forever by `formula_cache`.
  `CellId::Create(kind)` is a records-mode `+`/`+cat` column (improvise-poia),
  where what the user types is a **name**, not a value. And `None` is **not**
  any of those — it is "no projected cell here": a padded slot, a blanked one,
  or a records-mode column that renames a coordinate across the whole row rather
  than writing one.
- **The selection is absolute and the grid is not.** `GridCache` is indexed by
  position *within the viewport*, while `selected` and `row_offset`/`col_offset`
  are positions within the whole grid. Reconciling the offset is the entire
  conversion, and the trap is that forgetting it looks right at the top of a
  document and wrong after the first scroll. `render.rs` does it by *adding* —
  `r + cursor.row_offset == sel_row` — so there is no underflow path and a
  selection above the window simply never matches.
- **The category panel is `flatten_cat_tree`**, not a second walk of the cache.
  That function exists in `improvise-protocol` precisely so this end and
  `move-panel-cursor` share one flattening (improvise-edp); the page never sees
  `CategoryTreeCache` at all.
- **The grid's frame is `AxisFrame`, and every header decision is made here**
  (improvise-anr9). The multi-level header levels, the repeat suppression, the
  group runs and their collapse state come off the same `AxisFrame` methods the
  TUI widget calls, so the page receives labels already blanked where they repeat
  and group runs already spanned. Until this landed the protocol carried
  `GridLayout::row_label`'s slash-joined tuple and the page drew `Food/Alice`
  where the TUI drew two header columns — a derivation the TUI's own test says is
  wrong, shipped to the other front end.

Nothing in `RenderModel` is on the wire. It is a *projection of a projection* —
the client's own caches shaped for one renderer — so it may change freely without
a protocol conversation.

Four smaller decisions that would be re-derived wrongly:

- **`RenderRow` is an enum**, not a `label` plus an `is_group` flag, so a group
  header cannot carry cells and a data row cannot carry a collapse state. It is
  serialized with a `kind` tag because the consumer is JavaScript. `labels()` is
  **empty for a group header**, which carries its label in its own variant rather
  than in a one-element tuple — so a caller walking `labels()` over the whole
  model sees the data rows and nothing else, which is exactly what
  `neither_front_end_produces_a_slash_joined_label` wants. A caller that means
  "every label on screen" has to match on the variant.
- **`RenderColGroup::label` is an `Option`, not an empty string.** An empty
  string is a legal group *name* — `Category::add_item_in_group` accepts one, and
  the TUI draws its indicator like any other — so a sentinel would serialize that
  group identically to an ungrouped run and the page would drop its indicator
  (raised by Copilot on PR #23). `None` for an ungrouped run also keeps the runs
  a *partition* of the columns: the spans sum to the column count, so a page
  laying them out as `colspan` cannot leave a hole.
- **Column group runs coalesce by NAME alone**, which is what `src/ui/grid.rs`
  does (`group_name != prev_group`), and the two must agree. Collapse is
  category-scoped, so two adjacent groups from different categories may legally
  share a name and differ in it; keying on the pair would start a second run the
  TUI suppresses. The run's indicator is its FIRST group's, again as the TUI
  paints it. `col_groups` returns nothing at all when no column is in a group,
  because "this grid has no column groups" and "every column's group is blank"
  are different statements and the page draws a line for only one of them — the
  same asymmetry `GridCache::totals` has.
- **`hint` and `editing` are decided here, not in the page.** `hint` is
  `AppMode::hint_text()`, the same string the TUI's footer draws, so the page
  spells no keys of its own — which is what keeps a second front end from growing
  its own idea of the bindings, and makes the modes this client can *enter* but
  not yet *draw* (`Help` above all) state what they are instead of looking like a
  broken grid. `editing` is `AppMode::is_editing()` plus the `edit` buffer:
  without it, a browser that resolved `i` locally would change its mode indicator
  and then swallow every keystroke invisibly.
- **`minibuffer` is the same argument, for the other six text-entry modes**
  (improvise-fdt6). `editing` drives the *inline cell editor*, so it is scoped to
  the two cell-editing modes and reads the `"edit"` buffer by name — which left
  FormulaEdit, CategoryAdd, ItemAdd, SavePrompt, ExportPrompt and CommandMode
  opening a prompt and swallowing every character typed into it. `minibuffer` is
  `AppMode::minibuffer()` plus **the buffer that config names**, read with
  `improvise_protocol::read_buffer` so the `"search"` special case keeps its one
  home (gotcha 11); `None` is "no text-entry mode is open", so the page asks
  nothing about the mode. The colour stays semantic — `MinibufColor` crosses and
  the CSS class is the page's last translation, exactly as `draw.rs::minibuf_fg`
  is the TUI's (gotcha 26). It is drawn as a line of its own rather than *in
  place of* `#status` as the TUI's bottom bar does, because `#status` also
  carries the transport's failures and a repaint must not erase one (gotcha 48).

`RenderCell::edit_text` is the stored value, not the formatted one: seeding an
editor from `text` and committing turns `1200` into the string `"1,200"`. The
`numeric`/`error` flags are the server's answers, echoed — the client must not
re-derive them from the text, which is how the two ends drift.

`render_model` builds `header_labels(0)` **once for the whole frame**, because
the per-position accessors scan and asking them inside the row loop is quadratic.

---

### `render.rs` and the help overlay (improvise-ktv1)

`RenderModel` carries `help`, `help_page` and `help_titles`, and the decision
the page must not make is *whether the overlay is open*: an **empty `help` is
what says it is closed**. That is the shape `col_groups` already uses for "this
grid has no column groups" — an absent list rather than a flag the page has to
interpret alongside a mode it would then have to understand.

The rows themselves are `improvise_protocol::help::pages()`, the same ones
`src/ui/help.rs` draws. `RenderHelpRow` carries a `kind` string
(`heading`, `keys`, `accent`, …) which is a *semantic* name the page maps to a
CSS class — the `AxisColor → axis_fg` shape again, and the reason the page can
style a heading without knowing what makes a line a heading. Where the content
comes from and what is derived from the keymap is `context/code/protocol.md` §8.

## `web/app.js` — the page

The browser half of improvise-cr3, kept as thin as it can be made, now talking
to either of two peers (improvise-djm). **The boundary is JSON strings in and
out, so this file holds no protocol knowledge at all**: it never inspects a
`Command`, never builds a `CellId`, never decides whether a cell is editable or
whether a typed string is a number. Five calls are the whole harness:

| call | what it is |
|---|---|
| `client.set_viewport(rows, cols)` | the handshake frame, and every resize; also records the window so keys resolve here |
| `client.on_frame(json)` | fold one frame, in arrival order |
| `client.render_model()` | everything to paint, already decided |
| `client.typed_cell(r, c, s)` | the frame for "the user typed s here" |
| `client.on_key_event(k,c,a,s,m)` | the frame for a keypress, folded locally first where this client may answer it |

**The two rules this file must not break are ordering and framing**: frames are
folded one at a time in the order they arrive (a cell update that overtakes its
labels is dropped, not replayed), and the *first* frame out must be the
viewport, because the server builds the snapshot at it.

Everything in this file that is a *decision* rather than a paint now lives in
`web/shell.js` — `socketUrl`, the `#status` composition, the watchdog, the
extent arithmetic and `gridPlan` — so that `node --test` can reach it without a
browser. The sections below describe those rules; the code is one file over.

### Two transports, one client

The wasm bundle is the SAME artifact either way (improvise-xks6: one envelope
pair, two deployments). Only where the strings come from differs, so the
difference is confined to `openTransport` and nothing below it knows which peer
it has:

- **default** — an in-page Web Worker hosting the whole `App` over OPFS
  (improvise-9x6). No server process; this is what makes a GitHub Pages copy of
  the directory a working application.
- **`?ws=host:port`** — a websocket to `improvise-ws-server`. That process is how
  a document opened in the TUI is shared, so the path is kept; **the presence of
  the parameter is the whole selection mechanism**, because a second knob naming
  the same fact is a second thing to get wrong.

**Neither is reachable silently.** Every transport writes what it is doing to
`#status` — "starting", "connected", "cannot reach …", "disconnected" — so a
peer that never arrives reads differently from one that went away.
"disconnected" means exactly the second: a socket that opened and then closed. A
socket that never opened keeps saying "cannot reach …", because the close event
there is the tail of the failure and not news.

`#status` carries **two independent facts** — what the transport is doing and
what the document last said — so it composes them rather than letting a repaint
quietly erase a connection error.

### `socketUrl`: `wss://` is the default

A bare `host:port` gets `wss://`, because the address that works from where this
page is actually served is the one it should be easiest to write. The page ships
to GitHub Pages, which is https, and an https origin refuses a `ws://` address as
mixed content — so a `ws://` default would hand the browser a URL that cannot
work and report it as a connection failure, which reads like "the server is not
running".

An explicit `ws://` is honoured only where it can function: a page served over
plain http, which is the local `python3 -m http.server` case that pairs with
`improvise-ws-server` (which speaks plain `ws://` only — it has no TLS, so
reaching it from https means terminating TLS in front of it). From an https page
an explicit `ws://` is **upgraded** rather than passed through, since passing it
through can only produce a failure the user cannot act on.

### `set_viewport`, and the twin that is gone

Same frame, but it also **records** the window. A client learns its window only
from what it announced — the row frame's own length is the window clipped to the
grid, so the two differ exactly at the bottom edge, which is where the scroll
arithmetic matters. Without it the client cannot clamp, so every cursor key goes
upstream into `Host::handle`'s empty vec and the arrow keys silently do nothing
(improvise-bpq2).

There used to be a frame-only twin, `Client::viewport`, exported beside it and
called by nobody. Its doc said it was kept because `web/app.js` already spelled
it — and `web/app.js` called `set_viewport` in the same wave, so the reason was
false before it was read (improvise-seik). That matters more than an ordinary
unused function, because **the two are one letter apart and the wrong one fails
silently**: it sends an identical frame and simply forgets the window, which
turns every cursor key into the black hole above. It is deleted, along with the
exported `init()` free function that duplicated `new Client()`.

A deletion is not the guard, though — the next export with no caller arrives the
same way. `web/tests/bundle/wasm_surface.test.js` now reads the generated
`.d.ts` and requires every declared method to be one `app.js` calls, or to be
named in an `UNCALLED` table with its reason. `free` and `initSync` are
wasm-bindgen's own; `revision` is there because `on_frame` returns it. Proved by
injecting a spare `static viewport(...)` declaration into the built `.d.ts` and
watching it fail by name.

### `frameCost`: what the grid's own frame costs, in whole cells

One header row and one header column was right until improvise-anr9: the column
header is now one row PER LEVEL plus an optional group row, the row header is
`row_levels` columns wide, and inline group headers take a row each. Reserving
one of each announces a viewport the page cannot show, and `#grid` has
`overflow: auto`, so the DOM would acquire a scroll position of its own while the
client's cursor offsets were computed from the larger window — putting the
selected cell off screen.

It is **measured off the painted model rather than predicted**, and defaults to
the old one-and-one before the first paint, which is the only moment there is
nothing to measure.

**The totals strip IS reserved**, unlike the inline group rows below, and the
difference is what makes it countable: it is chrome on the same terms as a
header level — present exactly when `totals` is non-empty, once, never
interleaved with the data — so counting it is neither a feedback loop nor a
ratchet. The page bar is not reserved and needs no rule, because `#pages` sits
*outside* `#grid` and so is already absent from the box `extentOf` measures.

**Inline group ROWS are deliberately NOT reserved.** They take real height, but
how many are in view depends on how many rows the window asked for, so every way
of counting them is either a *feedback loop* (count the current ones: a smaller
window crosses fewer groups and asks for a larger one) or a *ratchet* (keep the
maximum: a group-dense grid stores an oversized cost on its first oversized
projection and never lets it fall — one group header per row clamps the grid to a
single row, permanently). Both were tried. What is left is an over-request
bounded by the groups in view: a grouped grid asks for a few rows more than the
table shows, which matters only for a selection near the bottom edge — and that
gap is not new and not specific to groups, since nothing in this page calls
`scrollIntoView` at all. Filed as its own ticket rather than answered with a
heuristic in a file the harness only partly executes.

### The watchdog, and the two `close` arms

**The handshake watchdog is about the *first frame*, not about either peer's own
failure modes.** The socket has an `error` event for a peer that never answers;
the worker has none that fires, because `worker.js` catches its own `start()`
rejection — so a browser with no OPFS, or a worker bundle that failed to load,
would leave the page saying "in-page worker" over an empty grid forever. "The
peer never answered the handshake" is the one condition that means the same thing
in both deployments, and a check aimed at each peer's own failure modes would have
missed exactly this one.

**A peer that failed *loudly* has already said something more precise, so the
watchdog must be DISARMED and not merely pre-empted**: it fires later and would
overwrite the better message with "did not answer" (PR #13 review). The watchdog
is for silence only, and `setShellFailure` clears the timer.

**Did this socket ever open?** The page has to record that itself, because
nothing on the close event carries it: `readyState` is already CLOSED by the time
a close listener runs, whichever happened, and a refused connection and a
mid-session network drop are BOTH code 1006 with `wasClean: false`. So "never
connected" and "was connected and went away" are distinguishable only by
remembering — an `opened` flag is the only discriminator.

A socket that never opened did not "disconnect": it failed to connect, and the
close event is the tail of that failure rather than a second, vaguer thing that
happened (improvise-taxx). Writing "disconnected" there unconditionally replaced
the `error` listener's precise message with the least informative of the two
facts.

**That is the same family as the watchdog bug and NOT the same fix.** The
watchdog is disarmed because it is for silence only, whereas suppressing `close`
outright would hide a genuine mid-session disconnect, which is the one thing that
listener is really for. Hence the split rather than a guard. **Both arms go
through `setShellFailure`, which disarms the watchdog** — and that half is the one
that keeps getting lost. It matters on the opened path: the watchdog is armed
after `transport.ready` resolves, i.e. on `open`, so a socket that opened and then
dropped before its first frame reaches the close listener with the timer still
pending, and "disconnected" is more precise than the guess it would otherwise fire
8s later. On the never-opened path there is no timer yet and the clear is a no-op.
Neither arm leaves one armed.

### Painting

- The **column-group line** is drawn only when there is one: an empty
  `col_groups` means this grid has no column groups at all, not that every group
  is blank. `label` is `null` for a run in no group, and an empty STRING is a
  legal group name that still gets its indicator — which is why the page does a
  null check and not a truthiness test.
- **One header line per level**, and a blank entry is *repeat suppression*
  decided in Rust, never an absent label.
- **The cell coordinates a click sends are DATA row indices**, and a group header
  is not a data row, so the page counts rows rather than trusting the position in
  `model.rows`.
- `r` is one binding shared by every closure in the row loop, so a click handler
  that captured it would fire with the LAST row's index. It is snapshotted.
- **A keyboard edit is open when `model.editing` is a string**, and it holds what
  has been typed so far. Both facts are decided in Rust — the page does not know
  which modes are editing modes or which buffer they type into. Without it the
  mode indicator would say INSERT while every keystroke vanished into a buffer
  nobody drew.
- **`editable` is the server's answer relayed through Rust**: a stored cell. An
  aggregate is a drill target and drill is not on the wire yet, so it is simply
  not offered.
- The inline editor seeds from `RenderCell::edit_text` — the STORED value, not
  the drawn one: `td.textContent` has been through the number format, so a stored
  1200 reads `"1,200"` and committing that stores the *string* `"1,200"`. Enter
  removes the focused `<input>`, which fires `blur`, which calls `close` again:
  harmless, but only by accident.
- **The title bar is three fields and one joining.** `doc_name`, `doc_dirty` and
  `doc_stored_as` come off the cache `src/draw.rs::draw_title` folds; the joining
  — `name (stored_as) [+]`, in that order — is `shell.js::documentTitle`, which
  is the TUI's own spelling minus the `improvise ·` chrome and the right-hand
  key strip. The three were projected by improvise-z754 and painted by nothing
  for a wave, and `render_surface.test.js` could not see it: that check is a
  **substring search**, so a field named by a function nothing calls reads as
  painted. What sees it is `contract.spec.js`, which loads the page and asserts
  the text (improvise-5jna). The marker itself is a node test rather than a
  harness one, because the standalone deployment persists on every accepted
  batch and clears the flag in the same round trip, so `[+]` is transient there.

### The selection scrolls itself into view

`web/app.js` called `scrollIntoView` nowhere at all, so a selection driven past
the bottom of `#grid` stayed there, invisible, with no way back but scrolling by
hand (improvise-jq8l). Two things about it are easy to get wrong.

**It is not a projection bug and not a `row_offset` bug.** The row had been sent:
`#grid` has `overflow: auto` and the page announces an extent from `ROW_PX`/
`COL_PX`, which **under-estimate** — a real row is 28.6px against the assumed 26
— and inline group rows are deliberately not charged for at all. So the page
renders more rows than its own box can show, and the overflow is the DOM's, not
the window's. Measured on `harness/fixtures/tall.improv`: 105px of overflow, the
selected cell 96px below the visible bottom, `scrollTop` still 0.

**`block: "nearest"` is the whole option.** It scrolls only when the element is
actually out of view, and it scrolls the nearest scroll container rather than the
document, which is what keeps the page itself from moving sideways.

The reason this was untestable until now is the fixture: `web/demo.improv` is
four rows and `harness/fixtures/grouped.improv` is eight, and neither overflows a
484px box. `tall.improv` is thirty rows in three groups, and the spec asserts the
overflow exists **before** asserting anything about the scroll — without that
line the whole test passes on a page that never scrolls.

### The pointer takes cells on `mousedown`

A cell's handler is `mousedown` with `preventDefault()`, not `click`, and the
reason is an ordering trap worth the paragraph (improvise-0x9u). From an
**editing** state a `click` never arrived at all: the mousedown moved focus, the
`<input>`'s `blur` ran `close`, `close` repainted, `replaceChildren` destroyed
the `<td>` the mousedown had landed on, and no `click` was ever dispatched. The
pointer was dead until the user pressed Escape or clicked outside the grid.

Three things had to move together, and each is load-bearing:

- **`mousedown`, so the handler runs before any focus change.**
- **`preventDefault()`, so no focus change happens at all.** Otherwise the
  default action focuses `#grid` *after* the handler has called `input.focus()`
  on the new editor, and blurs it immediately. The keyboard listener is on
  `window`, so nothing needs `#grid` focused.
- **`close` no longer repaints, and `blur` checks `closed` first.** Removing a
  focused `<input>` fires `blur` **synchronously**, so a `close` that repainted
  from inside itself detached the very `<td>` the handler was about to open an
  editor on. This is the same repair one level down, and it is why the first
  attempt at the fix still failed: the `blur` arm re-entered and rebuilt the
  table. `close` restores the **drawn** text rather than the seed, since nothing
  repaints behind it any more.

The state the old spec could not see is the whole point: it pressed Escape
before clicking the second cell, so it only ever clicked from a non-editing
page — this repo's characteristic defect, a test that passes while the thing it
names is broken in a neighbouring state.

### One handler per surface, and the node carries the answer

improvise-b8ot, on the user's ruling: *one handler outside the area that gets
redrawn, and dispatch events there*. The five clickable surfaces — the grid,
the category tree, the two side panels and the tile bar — had one listener per
**node**, created inside the render loops and reallocated on every repaint.

The roots were already stable and that is what made it cheap: `replaceChildren`
replaces an element's *children*, not the element, so every `el.*` entry
survives every repaint and needed no new wrapper.

**Not a performance change, and it must not be argued as one.** The cell count
is bounded by the announced viewport rather than by the document, and nothing
leaked — `replaceChildren` drops the nodes and their listeners together. The
argument is design-principles §5: paint and wire, applied to the dispatch as
well as to the paint.

**The operands are carried on the node, never re-derived from the DOM.**
`gridPlan` already decides `row`, `col`, `editable` and the class map, and its
`row` counts **data** rows because a group header is not one. A delegated
handler that walked up the table counting `<tr>`s would be a second derivation
of exactly that, and it would disagree the moment a group header appeared. So
the plan writes `data-row`/`data-col`/`data-edit`, `cellAction` reads them
back, and the handler is a lookup. The split follows the bead's own division:
**data attributes for operands**, the **existing `editable` class** for the
predicate, and **ids only for the surfaces**, which they already were — a row
or cell id would have to be unique per document and these are rebuilt per frame.

Four behaviours a naive rewrite loses, all silent, all asserted in
`harness/tests/contract.spec.js`: `mousedown` rather than `click` (above),
`preventDefault()` **and only over a cell** — a header carried no listener
before, so a blanket handler on the root is a behaviour change — the repaint
that is **skipped** when an editor opens, and closing a live editor **first**,
before the click is composed.

**The editor's own `keydown` and `blur` stay direct**, deliberately: one
transient node, one at a time, attached at creation and gone with it. `blur`
does not bubble, so delegating it would need a root that does not own the input.

### The page bar and the totals strip

improvise-p7lh, and the shape is worth more than the fix. `GridCache` has
carried `page_coords` and `totals` since improvise-n10; the server projected
both to every session, every client folded both correctly, the TUI painted
both — and `RenderModel` had no field for either, so the browser painted
neither. The shipped front page is `Quarter: page, Q1`, so it drew a grid
filtered to Q1 with nothing saying so, while `[`/`]` — which resolve *locally*
off that same `page_coords` — changed every number in silence.

**Why the parity guard did not see it.** `web/tests/render_surface.test.js`
asks that every field `RenderModel` exposes is painted. Its domain is
`RenderModel`, which is the thing under test: a fact that never reaches it is
outside the domain, so the check was closed under the defect. The honest
question is one level up — every field of every widget cache reaches a front
end, or is named as debt — and that is now asserted too. It would have fired
when the fields were added.

Two rules the painting itself owes. The **totals are relayed, never summed
here**: the total runs over every row including ones a viewport scrolled past,
which is why it is projected at all (gotcha 44), and an empty strip means "this
grid has no totals" rather than a row of blanks (gotcha 33). And the
**Page/Filter split is asked of `PageAxis::cycle`**, the one home of it: the
plausible alternative, `items.is_some()`, agrees on both common cases and
disagrees on an empty list, which is how a rule gets quietly respelled
(`invariant(the_page_filter_split_lives_in_the_type)`).

**Two residuals, named rather than left to be discovered.** The word `Total` is
now spelled twice — `src/ui/grid.rs` and `web/shell.js::gridPlan` — which fails
§5's own test, *would a third front end write this again*. It is kept local on
the `indicator()` precedent (the collapse glyph is spelled in both ends while
the *fact* is shared) and because the alternative is a protocol-level string
table for one word; `invariant(a_ui_string_is_keyed_by_a_command_never_by_a_key)`
is where it belongs if a second such string ever appears. And the page bar is
**display-only**: it shows the cycle position so `[`/`]` have an affordance, but
clicking an entry does nothing, because composing a page change from a click
needs a `click_page` on the wasm surface that does not exist yet.

### The keyboard

`Client::on_key_event` classifies the event and folds it locally where this
client may answer it — text entry, and since improvise-bpq2 the grid, panel and
tile cursors too, which is why `sendViewport` must call `set_viewport` and not
`Client.viewport`. Anything it cannot answer becomes a `ClientFrame::Key`, which
`Host::handle` answers with an empty vec: those keys are sent and do nothing.
**That is settled rather than unfinished** — the host must not know the keymap —
and closing one means teaching the client, not that arm.

The page does not claim a key it did not answer: a frame means the page claimed
it, so the browser must not also act on it — an arrow that both moves the cursor
and scrolls the page is the worse of the two errors. Combinations under Ctrl or
Alt are left alone regardless: those are the browser's own shortcuts, and the
client resolves nothing under them. `BrowserKey` drops anything held with the
platform meta key precisely so `Cmd-R` does not arrive as a bare `r`; the page
does not claim it either.

### The deployment notice, and the one thing this page remembers

`#backend-notice` is a `<details>`. The collapsing is therefore the **browser's**
— keyboard operable, and correct with `app.js` absent, still loading, or thrown
— and the only thing the page adds is remembering the answer. That split is the
whole design: a static notice is what it was, and a notice whose open state is
decided in JavaScript would be a fourth thing that can be wrong on a page whose
job is not to be wrong about its own peer.

**It starts collapsed for a visitor who has never answered**, and the argument
is that collapsing withholds nothing. The one fact a first-time visitor needs —
the document lives in this tab, is saved to this browser's private storage, and
never leaves the machine — is the `<summary>`, which never collapses. What is
behind the disclosure is `?doc=`, `?seed=`, `?ws=`, the `wss://` default and the
reverse proxy a plain-`ws://` server needs: *configuration reference*, looked up
rather than read, and irrelevant until a visitor has decided they want a second
document or a shared one. Expanded, it cost about 230px of an 820px window and
pushed the tile bar, the category tree and the grid below the fold on every
visit. The page is an application; it should open as one.

**The state is read through `shell.js` and stored in `app.js`**, on the usual
split: what a stored value *means* is a decision and is `noticeOpen`, while
touching `localStorage` is a browser API and stays in the page. `localStorage`
**throws** in a private window and in a context with site data blocked, and
returns `null` having simply forgotten, so every access is wrapped and there are
exactly two recognised spellings. Everything else — absent, empty, a value an
older page wrote — is *unanswered*, which is the default rather than a third
state: a page that treated forgetting as its own case would have to decide what
to do about it, and there is nothing to decide.

What is stored is per-viewer and per-origin by construction. It never reaches
Rust, never crosses to a peer, and is not state the application has any opinion
about — which is exactly the class browser storage is for, and exactly why it is
not a `ViewState` field (gotcha 30).

**The page's own keydown listener must not claim the summary's keys.** `Enter`
is claimed by the client (it is forwarded upstream), and the listener calls
`preventDefault()` on anything claimed — so without an explicit guard the
disclosure would open by pointer and be dead by keyboard, which is the whole
reason for using `<details>` rather than a `<button>` and a class. The guard is
the same shape as the inline editor's: the control that has focus owns its keys.

### Where the minibuffer is drawn, and why that is not a second derivation

The TUI draws the minibuffer on the bottom line, *instead of* the status line.
The browser has always drawn it in its own element, because `#status` carries
the transport's failures and a repaint must not erase one (gotcha 48) — but it
drew it as a 14px line in the page's bottom-left corner, which in a terminal is
a convention with decades behind it and in a browser window is where nothing is
read. With the grid scrolled, the cell being typed into can be 800px from the
text showing what is being typed.

So it is now a raised bar: fixed, centred over the grid, larger, bordered in the
mode's own colour and shadowed. **Every part of that is paint.** The content is
`RenderMinibuffer`, built in `render.rs` from `state.view.mode.minibuffer()` —
the same semantic value the TUI reads — and the colour is `MinibufColor`
resolved to a concrete colour exactly as `draw.rs::minibuf_fg` resolves it for
the terminal. Nothing here asks which mode is open, which buffer is being typed
into, or what the prompt says; a second answer to any of those is the disease
design-principles §5 is about. The caret is drawn as a CSS `::after` rather than
appended to the text for the same reason: the text is the host's, and the page
adds nothing to it.

`pointer-events: none` is load-bearing — the bar floats over `#grid`, and a
prompt that ate a click meant for a cell would be a new bug introduced by paint.

### What the styling is, and what no test here can see

`web/style.css` stopped being "legibility only" and is now a small design
system: layered surfaces, one accent, a filled badge for the mode (the single
most consequential fact on the page, since every key means something different
in each), chips for the two axis strips, and one selection treatment used
everywhere a selection can occur. The palette is deliberately the landing page's
(`docs/index.html`), so the two read as one product.

Two things in it are worth naming because they look like derivations and are
not. The selected row's header lights up through `tbody tr:has(td.selected)`,
which is the *selection already decided in Rust* being read off the DOM by the
stylesheet rather than the page working out which row is selected. And
`.notice-toggle`'s glyph is chosen by `[open]`, which is the one piece of state
on this page that genuinely is the browser's and that nothing else knows.

**The harness cannot see any of this**, and that is the honest scope. Playwright
asserts what a browser can answer: that the notice starts collapsed and
remembers, that it opens from the keyboard, and that the minibuffer is inside
the viewport, overlaps `#grid` and is larger than body text. It cannot see
colour, contrast, spacing, weight, the grouped or multi-level header DOM, or
either theme — so the rest of the styling is **argued and not tested**, on the
same terms as the rest of this section, and a change to it is reviewed by
reading and by looking at the page.

---

## `web/worker.js` — the worker half

It hosts the authoritative `App`. Everything it decides is decided in Rust —
the file only moves strings between `self.onmessage`/`self.postMessage` and
`WorkerServer::on_message`, which is the same shape `web/app.js` has on the main
thread and for the same reason: the less JavaScript there is, the less can be
silently wrong.

**Two rules it must not break:**

1. **Post every reply, in the order the Rust returned them, one message per
   frame.** A `GridUpdate::Cells` that overtakes the labels framing it is DROPPED
   by the client's fold, not replayed. Never batch two calls' replies together
   and never reorder them.
2. **Await `flush()` after each message.** `Storage::delete` is visible to
   `read`/`list` immediately but only reaches OPFS there.

**Rule 1 is about frames within one call's replies AND about the calls
themselves**, which is why every message is chained onto one tail promise rather
than handled by an `async` listener: **an `async` listener does not serialize**.
The event loop is free to start the next `message` while an earlier one is
suspended at an `await`, so two `on_message`/`flush` pairs interleave and a later
frame's replies can be posted before an earlier frame's. The chain is also what
makes "frames that arrived before `start()` finished" an ordinary case of the
same rule instead of a second code path with its own drain loop — and that loop
had the race in it twice over, since `server` was set before it ran.

The chaining itself is `web/serial.js::serialize`, which is promise plumbing and
nothing else, so `web/tests/serial.test.js` can force the interleaving a browser
cannot be asked to produce on demand — including the negative arm, which runs
the same two handlers *without* the chain and shows them coming back swapped.

**The document is named in the worker's own URL** — `new
Worker('worker.js?doc=demo.improv')` — the analogue of the socket server naming
it in the `ws://` path. It is deliberately not a `ClientFrame`: the envelope pair
is shared with the socket deployment and stays exactly as it is (improvise-xks6).
The seed is fetched and handed to Rust, which **writes** it through the VFS and
then reads it back like any other document — not injected past storage. A missing
seed is not an error: with none, the worker creates an empty model under `DOC`.

An unhandled rejection inside a `message` listener reaches the devtools console
and nothing else, so it is caught there — naming which frame it was, which the
main thread cannot see and has no way to ask — rather than left on the chain,
where one bad frame would poison every later one.

---

## `docs/` — the landing page and its casts
### The casts are recorded by a script, not by `asciinema rec`

`docs/record-casts.py` drives `improvise` on a pty it owns and writes asciicast
v2 itself. `asciinema rec` records a **shell**, and the shell it records is the
recorder's own: every previous cast opened and closed on
`(0) Mac:<hostname> ~gf/u/edwlan/improvise`, on what improvise-kh8 had since
made the site root, and `formulas.cast` and `import.cast` showed that prompt as
their visible **opening frame** (improvise-2oxt).

The repair is structural rather than careful: `improvise` is the pty's only
child, so there is no shell for a prompt to leak from — nothing to remember to
clean, and nothing that comes back when somebody re-records on a different
machine. Three other things stopped being accidents at the same time. The
**geometry** is an input (120×37, the numbers `docs/index.html` already spelled,
which is why they were not changed — a 30-row player over a 37-row cast clips
the tile bar and the status line, exactly where the output is). The **pacing**
is a per-step table rather than typing speed. And the header carries **no `env`
and no `timestamp`**: the first held `$SHELL`, the second the recording
machine's clock, and neither is anything a visitor is owed.

Two decisions inside the scripts that are not obvious and cost a re-record each:

- **No `:q` is typed at the end.** The player loops, so the last frame is also
  the frame before the first, and a minibuffer reading `:q` is what it would
  show. The process is signalled instead.
- **Each scenario has to be arranged so the thing it names is visible.** The
  drill cast drilled into a cell with no records behind it — a true frame about
  nothing — so it now moves `Customer` off the page axis first, which makes the
  cell an aggregate over nine customers and gives the records grid more than one
  row. The pivot cast pressed `c` on a category that was already on the column
  axis, which is a no-op with a status line. Neither was visible from the
  keystrokes; both were found by rendering the recording back and reading it.

That last point is the method, and it is reusable: a cast can be replayed
through a small ANSI reconstruction and read as text, so "what does frame N
actually show" is a question with an answer rather than a guess. It is how the
`+cat` and `+` columns in `drill.cast` — improvise-2oxt's substantive
complaint, since the old recording predated both — were confirmed present.

What the harness can check, and does: `landing.spec.js` asserts over the cast
**files** that no header carries `env` or `timestamp`, that the geometry is the
one the page configures, and that no chunk contains an OSC window-title
sequence, which is the leak's signature — `improvise` emits none and a shell
emits one per prompt, carrying the cwd. Asserted over the file rather than the
rendering because that is where the leak lives: a player draws a prompt as
ordinary text, and a screenshot assertion would have to know what the hostname
had been.

---

## `harness/` — the browser instrument

### Why it exists, and what belongs in it

Before `harness/` there was no JavaScript test infrastructure in the repository
at all — no `package.json`, no runner, nothing in CI running a single line of
`web/app.js` or `web/worker.js` — so every bug in them was found by reading and
the bug-first rule could not be honoured for a change to either. **PR #21
declined to add regression tests for its close-listener fix on exactly that
ground, and that decline is only honest because those assertions live here
instead.**

Playwright rather than a headless-DOM runner because what goes wrong in these
files is a real Worker, a real WebSocket, real OPFS and real event ordering, none
of which a DOM shim has. That still holds, and it is why no shim was added; what
changed is the **scope** — see the section below, which is now the answer to
"where does a change to `web/*.js` belong". It is deliberately small (improvise-vvl3): the point is
to be able to run `storage_contract` against `OpfsFs` and to read `#status` out
of a real page, not to grow a second test framework beside `cargo test`. The
server root is `harness/serve`, built by `build.sh` — the assembled site plus the
harness's own page; a module script will not load from `file://`, so an http
server is not optional.

### Engine coverage

improvise-xf1's acceptance criterion names Chrome, Firefox and Safari, and the
OPFS spec is the reason: `createSyncAccessHandle` is the one API the whole design
rests on, its sync read/write/truncate signatures were added later than the
handle itself, and each engine implemented it separately. Chromium alone would be
one engine's opinion.

**Result today: Chromium and Firefox run the whole battery and pass.**
Playwright's WebKit provides **no OPFS at all** — `getDirectory()` itself rejects
with `UnknownError` — so Safari is NOT covered and xf1's criterion is met for two
engines of three. The project is kept rather than deleted so that it starts
running the day that build gains OPFS; `opfs.spec.js` skips on a **runtime
probe**, not on the browser name, so if Chromium or Firefox ever stops providing
OPFS it fails rather than skipping. An unexpected skip on any other engine is a
failure — a silently skipped check is exactly what this harness exists to
prevent.

The shell spec's **mocked-peer** block is Chromium-only on purpose: it is about
`web/app.js`'s own logic, which is engine-independent, and `routeWebSocket` is
slower and flakier to no benefit across three. That argument is about those four
tests and was never made about the two beside them: the **standalone** block
mocks nothing, it is the deployment improvise-bck's criterion (1) names in so
many words ("Chrome and Firefox"), and the engine's OPFS is already exercised by
`opfs.spec.js`, so the path is known to be there. It runs on Firefox too since
improvise-zkkw — `testMatch` widened, `grepInvert: /@mocked-peer/` — measured at
**2.0s** for the pair and 10/10 green under `--repeat-each=5`. The split is a
tag rather than a third copy of the file, and widening the whole file and then
quieting what flaked would be adopting a reason nobody gave.

Every other spec is Chromium-only, including the three added by improvise-plcx,
78v6 and bgtf: a fixture document, a help overlay and a landing page decide
nothing per engine.

### `harness/opfs-contract`

A *harness* crate, not part of the application: it ships in no bundle and is not
a workspace member. `OpfsFs` had been compile-checked and never run; this is the
instrument that changed that (improvise-vvl3, absorbing improvise-xf1).

**Why a worker.** `FileSystemFileHandle.createSyncAccessHandle()` exists only in
a dedicated Web Worker, and it is the whole reason `Storage` could stay
synchronous. A check that ran on the main thread would fail for a reason that has
nothing to do with `OpfsFs`.

**Why a report rather than a panic.** Every check runs and its outcome is
reported, so a failure names *which* step and the rest still run. More
importantly it is what lets the spec check the checker: Playwright asserts the
report contains exactly the checks it expects, so a worker that never started, or
a build that dropped a check, is a failure rather than a silent pass. The list of
expected check names is spelled in `opfs.spec.js` **as well as** in Rust on
purpose: a check deleted on one side has to be deleted on both, which is what
stops the battery quietly shrinking.

**`wipe` uses only the backend's own vocabulary** — `refresh`, `delete`,
`commit_deletes` — rather than reaching for `removeEntry` directly, so a broken
`delete` shows up there as well as in the contract. It therefore cannot clear a
zero-byte entry, because `list` no longer names one. That is deliberate and costs
nothing: `attach` recreates and `write` fills whatever a previous run left behind,
and a leftover empty entry is invisible to every assertion for the same reason it
is invisible to a document picker.

`opfs_root`/`make_directory` exist only in the harness: `OpfsFs` has no directory
vocabulary at all — it stores one flat entry per document — so making a directory,
to check that `refresh` skips one, has to happen outside it.

---

## Where a change to `web/*.js` is tested

**E2E and integration tests are rare and pin boundary contracts; unit tests
assume those contracts and test between them.** (User ruling, 2026-09-20.) So
the question is not "can this assertion be made cheaper" but "which boundary is
it about", and the browser half has seven:

| boundary | the contract | where it is pinned |
|---|---|---|
| `RenderModel` → DOM | every field the render model projects reaches an element | `web/tests/render_surface.test.js` (statically) **and** `harness/tests/contract.spec.js` (on the screen) |
| JS → wasm | the calls the page makes exist, with the arity the bundle declares, and their answers are painted | `web/tests/bundle/wasm_surface.test.js` (the surface) **and** `contract.spec.js`'s pointer test (the wiring) |
| page → peer | the handshake goes out first, a frame is answered whole, a failure is named, **and the page routes each failure through the call that disarms the watchdog** | `harness/tests/shell.spec.js`, `harness/tests/drill.spec.js` |
| key → screen | a key the mode's hint line ADVERTISES produces something the user can see, and a text-entry mode paints what is typed into it | `harness/tests/keys.spec.js` |
| `help::pages()` → DOM | the overlay's rows are the shared content — the keys off the keymap and the words beside them — and not the right number of blank rows | `harness/tests/help.spec.js` |
| worker → OPFS | `storage_contract`, run against a real `OpfsFs` | `harness/tests/opfs.spec.js` |
| `AxisFrame` → DOM | the grid's *frame* — header levels, group spans, the collapse indicator and the group row that carries no cells — is drawn as the frame says, not slash-joined | `harness/tests/grouped.spec.js`, over `harness/fixtures/grouped.improv`; `web/tests/grid_plan.test.js` pins the plan it draws from |
| the site → the app | the landing page's casts are really fetched and really play, and its front door reaches `./app/` | `harness/tests/landing.spec.js` |

**A unit test of a rule does not test the wiring that reaches it**, and the
sharpest case in this file is the watchdog. `shell.test.js` proves that
`createShell.fail()` disarms rather than pre-empts; it cannot prove that
`app.js`'s socket `close` and `error` listeners call `fail()` rather than
`setShell()`. That routing is PR #21's other half, the one gotcha 48 says keeps
getting lost — and PR #61 lost it a third time by keeping the unit test and
dropping the browser assertion, with 26/26 and 13/13 green. Restored, and the
rule generalises: **when a unit test replaces a boundary test, ask which of the
two the regression would have been in.** Here it is the routing, so the guard
stays in Playwright.

**The faked clock is faithful outside the window and silent inside it.**
`shell.spec.js` walks past the 8s handshake watchdog with `page.clock`, and the
question that owes an answer is whether collapsing ten seconds into one
synchronous step *flattens* an ordering rather than merely skipping the wait.
Probed for the sharpest case — a peer that opens, stays silent past the window
and is closed afterwards — the `#status` transcript is **byte-identical** under
the fake clock and under a real nine-second wait: four states, same order, same
text. What that does not cover is an event landing **strictly inside** the
collapsed window, which a synchronous `runFor` gives no opportunity to deliver;
that is a race whose outcome the page does not promise, so there is nothing to
assert. State it rather than re-derive it.

**A negative assertion must demonstrate the denied event is reachable on that
exact code path** — not on a sibling path, and not in the rule's own unit test.
This file's worked example cost four review rounds. `shell.arm` runs only after
`await transport.ready`, so on a socket that never opens the watchdog is never
armed; a test denying "sent no document" there denies nothing, and passed
against both the routing revert and the disarm regression. `outlastTheWatchdog`
now asks the page `timesArmed() > 0` first, which turns that review finding into
a failure by name. Note *ever armed* and not *armed now*: a correct page has
already disarmed by the time the disconnect test jumps — that is the property —
so the obvious spelling would have denied the passing case.

**And a constant a test must exceed has one home.** `HANDSHAKE_TIMEOUT_MS` lived
in `web/app.js` and again in the spec, under a comment saying a silent
disagreement "would pass for the wrong reason". It did: the spec at 1000 against
the page's 8000 made the suite green over the reverted routing, because the jump
landed inside the window. It is exported from `web/shell.js` and imported by
both, so the disagreement is unconstructible rather than discouraged.

Everything **between** those seams is `node --test web/tests`, and it may assume
the boundary holds rather than re-establishing it: transport selection from
`?ws=`, the two facts `#status` composes, the watchdog's arm and disarm, the
`frameCost`/extent arithmetic, the collapse glyph, the grid's plan, and the
worker's tail-promise ordering. None of it needs a DOM, so none of it is
Playwright's.

**The seam that makes this possible is `web/shell.js` and `web/serial.js`**:
pure modules that **return a description** rather than take a DOM double.
`gridPlan(model)` answers every question `gridTable` used to answer while it was
creating elements — which row index a click carries when group headers are
interleaved, what a group header spans, whether a `null` group label is a name,
whether an empty edit buffer means an editor is open — and `app.js` is then a
loop that creates elements from it. Painting stays untested by Node; the
decision about *what* to paint stopped being untestable.

### Why the four widgets were one bug

The tile bar, the view list, the formula panel and the category sidebar
ignoring `category_panel_open` were reported as four broken features and fixed
as four. They were **one failure at one boundary**: a `RenderModel` field with
no element painting it. Four per-feature specs cost four page loads and catch
the fifth only if somebody writes a fifth spec; one assertion that *every*
projected field is painted catches it by construction. That assertion is cheap
enough to make twice, from different sides:

- **`render_surface.test.js`** reads the `pub` fields off
  `improvise-wasm-client/src/render.rs` and requires each one to be read by
  `app.js` or `shell.js` — or named in `UNPAINTED` **with a reason**. It needs
  no browser, no bundle and no build, so it fails in seconds. The list is not
  an escape hatch: it is the debt, written down, and it currently holds
  `doc_name`/`doc_dirty`/`doc_stored_as` (improvise-5jna — the fifth instance,
  found by writing this test).
- **`contract.spec.js`** drives the demo document into each state that fills a
  widget and asserts the element is *on the screen*. Node cannot see that, and
  the moment a reduction would have to weaken it, it stays here.

**And the static half is weaker than it reads, which is worth knowing before
leaning on it.** `render_surface.test.js` asks whether the string
`model.<field>` occurs anywhere in the page's code — so a field *mentioned* by a
function nothing ever calls passes. That happened on improvise-fdt6: the new
`minibuffer` field had a `paintMinibuffer` reading it, and the call site and the
`getElementById` were both missing, so the node suite went green over a page
that still painted nothing. `keys.spec.js` failed on its first run, in Chromium,
naming the hidden element. The static check is a **completeness** check — it
tells you a field was not forgotten — and never an **execution** check; only
`harness/` is that (gotcha 48).

What is deliberately **not** in Playwright any more is what each key or click
*means*: `improvise_protocol::resolve` decides that and its tests are where the
meaning is pinned. One instance of "a resolved change repaints" is kept, because
the repaint is the boundary; the other seven were the same instance again.

### Two numbers, and neither is the other

Measured on one machine, in one session, on a private
`IMPROVISE_HARNESS_PORT`, with **both trees clean** — `1af51c9` reached by a
full checkout rather than by restoring paths over a working branch (see the
trap below). Two runs each; the pair is the variance.

| suite | chromium | all projects | execution | whole job |
|---|---|---|---|---|
| Playwright, before (`1af51c9`) | 23 | 26 passed + 3 skipped | 36.2s / 36.3s | ~4m30s in CI |
| Playwright, after | 14 | 17 passed + 3 skipped | 15.1s / 14.9s | ~4m30s in CI |
| Playwright, + wave 4 | 21 | 26 passed + 3 skipped | 34.4s / 33.7s | ~4m30s in CI |
| `node --test web/tests` | — | 26 | 0.12s | seconds |

The last row is improvise-78v6/igxp/plcx/bgtf/zkkw: seven more Chromium tests
and two Firefox ones. **Most of the 19 seconds is one test** — the landing
page's four asciinema casts are played until each terminal draws, which is the
only assertion that can tell a cast from a 404 (evidence file). The Firefox pair
is 2.0s of it.

**Two columns because one number was ambiguous three times.** Firefox runs
`opfs.spec.js` and the standalone half of `shell.spec.js` and every other spec
is Chromium-only, so a count taken off a run is five larger than the chromium
count, with three more skipped (WebKit's OPFS battery). The columns must also **agree**: chromium + 3 is the passed total,
and a row where they do not is a row built from two different trees.
`npx playwright test --list` prints the per-project truth; quote which one, and
this is the same "two numbers, not one" the Rust suite learned in
`design-principles.md` §7.

**The trap that made the first `before` figure wrong.** It was taken by
`git checkout 1af51c9 -- harness web` inside a working branch — which restores
*tracked* files and leaves untracked ones **where they are**, so the branch's own
`contract.spec.js` stayed and the "baseline" ran main's 23 tests **plus three of
the branch's**. A baseline taken by checking paths out over your own work
measures a tree that never existed. The full checkout above is the repair.

**What gave it away was the count, not the clock — and that is the lesson.** The
contaminated run read 29 passed where a clean `1af51c9` reads 26, because three
extra tests really were there; but it read 36.9s against the clean tree's 36.2s,
because those three cost under a second between them. **A wall clock absorbs a
contaminated tree without a trace; an arithmetic identity does not.** The same
error survived three separate corrections — it reached PR #61's body as "26
chromium" (23 + the 3), survived a fix that corrected the chromium column and
left the run count beside it, and was caught each time only by someone checking
that the columns add up.

**Where the saving actually came from, since the obvious answer is wrong.** The
eleven per-widget tests that were deleted cost **2.7 seconds of test time
between them** — restored verbatim and re-measured — against **1.55s** for the
three contract tests replacing them. Those are the per-test sums, not the two
wall clocks, which read 3.3s and 3.4s and **flattered the replacements**: they
were separate `npx playwright test` invocations, so each paid its own cold
start (~0.6s and ~1.9s) and a near-tie appeared between two things not measured
the same way. Warm inside a full suite those three cost under a second, which is
why the contaminated baseline's clock barely moved.

Eleven tests at 2.7s cannot buy a twenty-second saving, and they did not.
Essentially all of 36.2s → 15s was two
`shell.spec.js` tests that **waited out the 8s watchdog in real time**, and one
of those two was the routing guard this section explains should never have gone.
So the deletion of the eleven bought shape, not speed; the speed came from a
deletion that cost coverage. Both are back at ~75ms each, because `page.clock`
fakes the page's timers — the honest version of the trade: *the wait was the
cost, not the test.*

**Cutting Playwright tests barely moves the CI number** in any case, because
roughly four minutes of that job is three `wasm-pack` builds, `npm ci` and a
browser download — not test execution. The feedback-time win is the `js` job, which
needs **no Rust toolchain and no wasm bundle at all**, and that is a constraint
rather than an accident: the moment a `web/tests/*.test.js` file needs
`web/pkg/`, it inherits the four-minute build and the argument for it
evaporates. That is why the one test that does need the bundle lives in
`web/tests/bundle/` and runs in the browser job instead.

Until improvise-4ktt the mechanism was a directory convention and a glob, and
nothing checked it. `bundle_freedom.test.js` does now: it reads its own sibling
files and refuses one that names the bundle directory, and its companion
asserts `web/tests/bundle/` still holds a test that *does* — a scoping rule is
uninteresting if nothing falls outside it. Evidence in
`context/test-evidence/tooling.md`.