docs: horizontal plot split

docs/claude/probe-readings.md

@@ -424,13 +424,23 @@ Two lines of the self-check output are assertions in disguise:
   the group, because `hold_pinned_window()` prefers a member that has moved since `pinned_stated_`
   — that is how it tells a gesture from its own echo — so a fit that wrote the group alone reads
   back as the zoomed range one frame later.
-- `[layout-probe] <action> rows=N buttons=B done=D done_box=[x,y,WxH] rows_y=[..] close=[..] add=[..] traces=[..]`
+- `[layout-probe] <action> rows=N buttons=B done=D done_box=[x,y,WxH] rows_y=[..] close=[..] add=[..] traces=[..] cells=[..] colweights=[..]`
   `side_w` is the sidebar column's width and **must be its own request (280), never 0**: it read 0 while
   the divider sat at the window's right edge, which is a panel that is not there. It was 280 during the
   first frames and 0 once the window had settled, so read it from a `placed` line rather than the first.
-  (stderr, `UDPSCOPE_PROBE_LAYOUT_MS`) — **`buttons` must be `2N+1`** while the editor is
-  up (one `×` per row, one `+` per boundary) and **0 on the `editor off` line**: the
-  buttons exist only while the editor does. `done` follows the same rule for the editor's
+  (stderr, `UDPSCOPE_PROBE_LAYOUT_MS`) — **`buttons` must be `3P + 2F + 2`** while the editor is
+  up, for `P` plots in `F` figure rows: one `×` per plot (`P`), one row `+` and one row resize arrow
+  per row boundary (`2(F+1)`, the arrows of the figure's two outer edges counted although hidden), and
+  per row of `n` cells `n + 1` cell `+` buttons and `n - 1` column arrows (`3P` in all, since
+  `sum(n+1) + sum(n-1) = 2P`, plus the `P` of the `×`). With no split that is **7 for one row and 12 for
+  two** (`5` and `8` before the split, the old `2N+1` having long stopped counting the arrows), and a
+  split row adds three per extra cell. **0 on the `editor off` line**: the
+  buttons exist only while the editor does. `rows=` is the plot count and `cells=[..]` the cells of each
+  figure row, comma-separated (`[1,1]` for two unsplit rows, `[1,2]` for a split second row);
+  `colweights=[..]` is each figure row's cell weights, `|` between rows and **empty for a row with none**
+  (every unsplit row, and a split row nobody dragged), so two unsplit rows read `[|]` and a dragged
+  `{3,1}` second row `[|3,1]`. An unsplit run must read `cells=[1,..,1]` and a `colweights` of
+  only separators; anything else is a split the run did not ask for. `done` follows the same rule for the editor's
   own exit button (1 up, 0 off), and its `done_box` on the `placed` line must sit in the
   area's top-right corner: `x + W` within a few pixels of the area width and `y` within a
   few of 0 (the on-switch line reports a box from before the first frame, so read the
@@ -661,7 +671,7 @@ Two lines of the self-check output are assertions in disguise:
     than by the first-run timer.
 - **The session layout round trip** (`~/.config/udpscope/<host>_<port>.toml`) — run once
   with a drop and a window change, then again with no interaction: the second run's dump
-  must read `layout loaded from ...` and `layout: rows=R window_s=W row0=[...]` with the
+  must read `layout loaded from ...` and `layout: rows=R window_s=W row0=[...] ... cells=[..] colweights=[..]` with the
   first run's values, and the signals it names must come back *acquired* (a `row 0 slot 0
   -> channel ... samples=N` line). Delete the file before a run whose layout you did not
   mean to inherit (`127.0.0.1_9540.toml` and friends accumulate otherwise). A **persistence
@@ -2838,3 +2848,62 @@ any is a badge that does not say its panel is open, or one that stays marked aft
 the plot's pan, drop and hover; `plot_unchanged=0` is the panel taking room from the plot. **Line 2:** `click_inside_closes=0` with `still_open=1` (a click
 inside the panel does not close it), `click_outside_closes=1` (a click on the plot does), `esc_closes=1` (`slideover_escape()` closes it) and `closed=1`.
 `esc_closes=1` is the function called directly; the key *through the window's handler* is the `esc:` reading of the `[keys]` line.
+
+## `[split-probe]`
+
+```
+[split-probe] start          plots=1 frows=1 cells=[1] widths=[756] tile=1 x_equal=1 cursors_aligned=1 weights=[-] traces=[1:sine,square,ramp,noise]
+[split-probe] split          plots=2 frows=1 cells=[2] widths=[376,376] tile=1 x_equal=1 cursors_aligned=1 weights=[-] traces=[1a:sine|1b:square]
+[split-probe] weights 3:1    plots=2 frows=1 cells=[2] widths=[533,220] tile=1 x_equal=1 cursors_aligned=1 weights=[3,1] traces=[1a:sine|1b:square]
+[split-probe] caps-calls      insert_cell=[1,1,0] refused_cell=1 insert_row=[1,1,0] refused_row=1 plots=8
+[split-probe] roundtrip       rt=1 expect=r0(h=2.0000)[w=1.0000:sine] r1(h=1.0000)[w=3.0000:square][w=1.0000:ramp] got=r0(h=2.0000)[w=1.0000:sine] r1(h=1.0000)[w=3.0000:square][w=1.0000:ramp]
+[split-probe] ruler-labels    split=[plot 1a,plot 1b,plot 2a,plot 2b] mixed=[row 0,plot 2a,plot 2b]
+```
+
+`UDPSCOPE_PROBE_SPLIT_MS` (stderr; see [verification.md](verification.md)). Nineteen steps, 250 ms apart;
+each restructures and the *next* step reports the frame that was laid out with it, so a line's label
+names the arrangement the line **reads**, not the call that made it. The state lines share one format:
+`plots` (`row_count()`), `frows` (`figure_row_count()`), `cells=[..]` (per figure row), `widths=[..]`
+(logical pixels of each cell, `|` between rows), `weights=[..]` (the stored cell weights, `-` for a row with
+none), `traces=[label:signals|..]` with the labels `plot_label()` gives (`1`, `1a`, `2b`). Every state line
+carries the same three assertions:
+
+| field | value it must read | the defect a wrong reading names |
+| --- | --- | --- |
+| `tile` | **1** on every state line | a row whose cells do not fill its rect edge to edge, are not at its y and height, or are not one `col_gap` apart, or a `FigureLayout` with `cells.size() != plots` / `rows.size() != frows`: the plotter's placement or a host that did not rebuild the figure |
+| `x_equal` | **1** | a plot whose x range differs from plot 0's: a cell that is not in the x link group (a rebuild that linked rows and forgot cells) |
+| `cursors_aligned` | **1** | a plot whose two vertical cursors sit elsewhere than plot 0's: a per-cell cursor time instead of the one shared time; the probe turns the cursors on and resets them first so the comparison is of real positions |
+
+The other lines, in order:
+
+| line | value it must read | the defect a wrong reading names |
+| --- | --- | --- |
+| `start` / `split` | `cells=[1]` then `[2]` with `widths=[376,376]` (equal halves of 756) and the traces on `1a`/`1b`; a second cell starts **empty**, the probe assigns it | a split that moved or dropped the first cell's traces, or unequal default cells |
+| `weights 3:1` | `widths=[533,220]`, **not** 3:1, and `tile=1` | not a defect: the plotter's 64 px floor per cell is taken off first and the weights share the remainder, so the shares read compressed; a result of exactly 3:1 would be the floor missing |
+| `row+cell` | `cells=[2,2]`, `weights=[3,1\|-]` | row 0's cell weights lost or moved when a row and a cell were inserted below it (they belong to the figure row) |
+| `caps-calls` | `insert_cell=[1,1,0] refused_cell=1 insert_row=[1,1,0] refused_row=1 plots=8` | a cap that is not reached or not held: row 0 starts with two cells and takes two more, refusing the third call at 4; two rows then fit before the plot total reaches 8 and the third is refused; `plots` must stop at **8** |
+| `caps` | `plots=8 frows=4 cells=[4,2,1,1]`, `tile=1` | a ninth plot, or a fifth cell in a row |
+| `remove` | `plots=7 cells=[3,2,1,1]` | `remove_cell()` on a multi-cell row that removed the row, or that dropped a neighbour's traces |
+| `roundtrip-io` | `wrote=1 read=1 plots_in_file=3` | a save that was refused, or a file that lost a `[[plot]]` table |
+| `roundtrip` (state line) | `cells=[1,2] widths=[756\|533,220] weights=[-\|3,1]` and the three traces on `1`, `2a`, `2b` | the arrangement did not survive `collect_layout()` -> file -> `apply_layout()` |
+| `roundtrip rt=` | **`rt=1`** with `got=` equal to `expect=` (row weights `h=`, cell weights `w=` and each cell's trace, read back at the file's `%.4f`) | **`rt=0`** names the difference: a weight written per plot index instead of per figure row, widths applied before the arrangement (which resets them), or a trace that moved to another cell |
+| `legacy-io` | `read=1 plots_in_file=3 row_unset=1` | the stripped file still carrying a `row`, which would make the next line test nothing |
+| `legacy` | `cells=[1,1,1]`, `frows=3`, `widths=[756\|756\|756]`, `traces=[1:sine\|2:square\|3:ramp]` | a session from before the split that did not open as one cell per row, or lost its signals |
+| `invalid-rows` | `ok=1 frows/plots=[5/5 3/3 3/3 3/3]` | one of four unusable `row` sequences (five cells in a row; a skipped row; a row that goes back; a first row that is not 0) that was refused, or kept as a split instead of loading one cell per row |
+| `editor-on` -> `cell-split` | `cells=[1]` then `cell-split-ck cells=2` and `cell-split` with `widths=[376,376]`, the new `1b` empty | the editor's cell `+` not reaching `insert_cell()`, or the figure not re-pointed after it |
+| `cell-assigned` | `1a:sine\|1b:square` | an assignment into the new cell that landed on another plot |
+| `col-drag` | `left_gt=1 both_above_min=1 left_gt_right=1 tile=1`; `pair_sum` constant (753 at the 756 px window) | the column drag not moving the boundary (`left_gt=0`), a cell dragged below `kMinCellPx` (`both_above_min=0`), or a drag that changes the pair's total instead of redistributing it. The probe applies +80 px, then +/-10000 px through `layout_col_apply()` |
+| `pre-insert0` (state line) | after the extreme drags `widths=[144,609]` | not a defect: the drag floors the left share at 96 px of the pair, but the plotter realises `64 + share * remainder`, so a cell dragged to the limit reads wider than 96 (see [invariants-plot-and-layout.md](invariants-plot-and-layout.md)) |
+| `insert0-move` | `move_ok=1 before=[sine,square] after=[,sine,square]` | an insert at the left edge that renumbered the cells instead of moving their traces right with them |
+| `remove-mid` | `rm_ok=1 after=[,square]` | removing a middle cell that dropped or shifted the wrong neighbour |
+| `row-weights` | `wt_ok=1 before=[3,1,1] after=[1,1]` | the row weights not following an insert or removal of a *figure row* (the vector had the plot-count length once and drifted off the rows) |
+| `sensitivity` | `cell_plus_sensitive=0 row_plus_sensitive=0 plots=8` | `-1` is a button the probe could not find; `1` is a `+` left clickable at the 4-cell cap (cell) or at 8 plots (row) |
+| `two-split-rows` | `cells=[2,2]`, labels `1a 1b 2a 2b`, all empty | `set_cells_per_row()` leaving the old plots' contents or labels behind |
+| `drop-rowat` | `ok=1 rowat=[1a=0,1b=1,2a=2,2b=3]` | `PlotArea::row_at(x, y)` choosing by y alone, which would put every cell of a row on its first plot: a drop or a right click on `1b` landing in `1a` |
+| `plot-on-buttons` | `[1a:row 1, cell 1 of 2\|1b:row 1, cell 2 of 2\|2a:row 2, cell 1 of 2\|2b:row 2, cell 2 of 2]` | a signals-page button labelled with the flat index, or without the cell tooltip; a one-cell row keeps its plain label and the tooltip "draw this signal on this plot" |
+| `assign-1b` / `drop-2b` | `assigned=1 notice='plotting 'sine' on plot 1b'`; `dropped=1 notice='plotting 'square' on plot 2b'` | a notice that names the plot by number alone |
+| `after-assign` (state line) | `traces=[1a:-\|1b:sine\|2a:-\|2b:square]` | the signal went to a neighbouring cell |
+| `ruler-labels` | `split=[plot 1a,plot 1b,plot 2a,plot 2b]` and, with cells `{1,2}`, `mixed=[row 0,plot 2a,plot 2b]` | the cursors pane's per-plot ruler line (a label read through the pane's own widgets): a split row's plots are `plot <label>`, an unsplit row keeps `row <plot index>` (**0-based**, as it always was -- `row 0` is the first plot, not a defect) |
+
+The run ends with the usual `gtk warnings: 0` and no `Gtk-CRITICAL`. `x_equal=1` and `cursors_aligned=1` on every state line are
+the only readings of the shared time window across cells.