Merge pull request #196 from fiddlerwoaroof/fix/projection-identities

improvise-exdg/7xyx: a coordinate cell is CellId::Coordinate; FrameGroup::cat deleted

context/code/protocol.md

@@ -614,14 +614,37 @@ broader than its reason — improvise-gcvb, which also records the two things
 that make it more than a one-liner: an inserted `+cat` column means the TUI
 does **not** advance, and `AddCategory` can abort.
 
+### `CellId::Coordinate` — a drawn coordinate is not "no cell" (improvise-exdg)
+
+A records-mode coordinate cell (the `Region` column holding `North`) used to
+project `None`. That was a **third** meaning of `None`, next to the padded slot
+and the blanked position: a visible cell that holds text and whose write
+belongs to the host. So absence was encoding a permission, the same conflation
+`Create` removed for the creating columns (improvise-poia). The pinning test,
+`a_coordinate_column_is_projected_with_no_identity`, asserted the bug and is
+gone.
+
+`Coordinate` carries **nothing**. Editing a coordinate renames it across every
+measure cell in the row (gotcha 1), and no client composes that operation: it
+needs `record_col`, and the browser declines every records-mode edit before
+reaching a cell. Giving the variant a `RecordHash` would hand a client a
+surrogate with nothing to spend it on. The operand belongs with the command
+that consumes it, whenever that command exists. Nothing a client can do has
+changed. `render.rs::editable` is now an exhaustive `match` rather than a
+`matches!`, so the next variant has to be classified. `typed_cell_command`,
+`record_at_cursor` and `record_cell_at_cursor` still answer `None` for it.
+So paste onto a coordinate is still a silent `Upstream` in the browser. The
+TUI has declined it out loud since PR #184. That divergence is
+improvise-kkcy, and a named kind is what lets the browser decline deliberately
+now.
+
 ### `CellDisplay::id` — `None` is not "aggregate"
 
 That is `Some(CellId::Aggregate(..))`, and the distinction is the whole point
 of the enum. `None` means the slot holds **no projected cell**: a `GridCache`
-position padded by `fit_to_labels`, blanked by a reframing, or a
-records-mode column that is not a cell to write into at all (a category column
-renames a coordinate across the whole row, gotcha 1, which is a different
-intent from `SetCell`). Conflating the two either hides a cell or offers a
+position padded by `fit_to_labels`, or one blanked by a reframing. Every
+records-mode column is some kind of cell (`Record`, `Derived`, `Create` or
+`Coordinate`). Conflating the two either hides a cell or offers a
 write the type system exists to make unconstructible.
 
 ### `CellDisplay::edit_text` — the difference corrupts data
@@ -637,8 +660,8 @@ and an editor's seed is exactly a feedback path.
 It is the projection of `App::display_value_at_cursor`, which is what
 `EditOrDrill` seeds the TUI's buffer from, so both front ends start from one
 string rather than each deriving one. `None` where there is nothing to edit:
-the same set of positions `id` is `None` for, plus cells whose value is
-absent. An error cell carries none, because its text is a diagnostic and not
+positions whose `id` is `None`, plus pivot cells with no key. A records-mode
+cell always carries one, even an empty one. An error cell carries none, because its text is a diagnostic and not
 a value.
 
 A browser found this: `tests/lockstep.rs` drove `i` through the host and the
@@ -785,9 +808,16 @@ one**. `AxisFrame::flat` leaves `cats` empty on purpose, so it is not a way to
 smuggle a multi-level axis in flattened.
 
 `FrameGroup::collapsed` is on the wire rather than left to the client because
-it is a fact about the *view*, which only the server holds. `FrameGroup::cat`
-travels so a client can echo it back in `ToggleGroup` without ever working out
-which category a label came from — which slash-joined labels made impossible.
+it is a fact about the *view*, which only the server holds. There is no
+`FrameGroup::cat` any more (improvise-7xyx). Its doc comment promised that a
+client echoes it back in `ToggleGroup`, but no client ever did. `resolve.rs`
+has no `toggle-group-under-cursor` arm (`z` is on the still-needs-the-host
+list), and neither renderer read the field. A promise about an interaction no
+code performs is the stale-sibling defect, so the field went with it. It is
+not a loss for `z`. The TUI's `ToggleGroupAtCursor` also places the cursor on
+the layout the toggle *will produce*, which a thin client cannot compute. So
+composing `z` in a browser needs its own design, and the operand should come
+back with that design rather than wait for it.
 
 ### `AxisFrame`: four accessors that are **not for a draw loop**
 
@@ -1294,13 +1324,13 @@ frozen behaviour reads as decided afterwards and nothing says it was only frozen
 for the duration of the move.
 
 **The ticket's own proposed fix does not work, and that is worth more than the
-fix.** h0iv suggested comparing the `(cat, name)` pair, `FrameGroup` carrying
-`cat` precisely so a client need not work out which category a label came from.
+fix.** h0iv suggested comparing the `(cat, name)` pair; `FrameGroup` carried
+`cat` then (improvise-7xyx has since removed it).
 But its own reachability case is *one* category: `Month` grouped into `Q1`,
 nested under `Region`, giving `North/Q1` and `South/Q1` adjacent — the two
 headers are `(Month, Q1)` and `(Month, Q1)`, **equal as values**, so the pair
-separates nothing. `the_two_headers_are_equal_as_values_so_comparing_cat_and_name_cannot_split_them`
-pins that, so the next reader does not re-propose it. What separates them is
+separates nothing. `the_two_headers_are_equal_as_values_so_comparing_them_cannot_split_them`
+pins that as whole-value equality, so the next reader does not re-propose it. What separates them is
 that they are two different `FrameEntry::Group` entries, and `groups()` already
 borrows each from `self.entries` — so the comparison is `std::ptr::eq` on the
 header, which is the literal statement of "the same header opened both".