Put a floor under the row-label wall

The last row-label column is where the deepest level's labels land once
the tree is opened, and nothing was sizing it, so expanding squashed
them against the numbers.

It is column group_by.length. The geometry is now written down, because
the old comment had it wrong and that error steered several failed
attempts: tree_header_offset is group_by.length + 1 and is where the
DATA starts, so the row labels are columns 0..group_by.length -- column
0 the indent gutter, 1..N the N levels. tree_header_offset - 1 is the
last label column, not a data column, and the hidden grand-total column
is the data one at tree_header_offset itself.

A constant rather than a measurement, because only the levels currently
on show have cells: at group_by_depth 1 the deeper label columns are not
in the DOM at all, so the wall is sized before there is anything to
measure. 218px was measured by hand on a rep/customer/month/part tree.

A floor, like the neighbouring 3-char one, so a wall dragged wider
survives the next layout apply.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/views/Forecast.jsx

@@ -1268,8 +1268,30 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
   // has to overflow its column, not the column grow to hold the text.
   const MIN_ROW_LABEL_CHARS = 3
 
+  // The wall: the last row-label column, which is where the deepest level's
+  // labels land once the tree is expanded to full depth.
+  //
+  // Its index is group_by.length, and the geometry is worth writing down because
+  // the comment on fitRowLabels had it wrong and cost a day. With four group_by
+  // levels the grid lays out rt-col-0..4 for row labels and starts the data at
+  // rt-col-5 (= tree_header_offset, group_by.length + 1). Column 0 is the indent
+  // gutter, and columns 1..N are the N levels — so the last level is column N,
+  // not N - 1. The hidden grand-total column is the *data* one at
+  // tree_header_offset, which the psp-split-total rule zeroes out; it is not
+  // this one.
+  //
+  // Only the columns for levels currently on show are rendered, so at
+  // group_by_depth 1 the deeper ones have no cells at all. Sizing this one by
+  // measuring the DOM therefore cannot work — it is sized blind, ahead of being
+  // needed, which is exactly why it is a constant rather than a fit.
+  //
+  // 218px was measured by hand: the width a rep/customer/month/part tree wants
+  // once it is open. This is the number to turn.
+  const ROW_LABEL_WALL_W = 218
+
   async function ensureRowLabelWidth() {
-    const grid = (await viewerRef.current?.getPlugin())?.regular_table
+    const viewer = viewerRef.current
+    const grid = (await viewer?.getPlugin())?.regular_table
     if (!grid?.saveColumnSizes) return
 
     const cells = [...grid.querySelectorAll('tbody th')]
@@ -1293,6 +1315,15 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     for (const idx of idxs) {
       if ((sizes[idx] ?? 0) < min) { sizes[idx] = min; changed = true }
     }
+
+    // A floor, like the rest: a wall someone has dragged wider is left alone.
+    const { group_by = [] } = (await viewer.save()) || {}
+    const wall = group_by.length
+    if (wall > 0 && (sizes[wall] ?? 0) < ROW_LABEL_WALL_W) {
+      sizes[wall] = ROW_LABEL_WALL_W
+      changed = true
+    }
+
     if (!changed) return
     grid.restoreColumnSizes(sizes)
     await grid.draw({ invalid_columns: true })
@@ -1319,11 +1350,16 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
   // the same circularity that stops headers sizing themselves.
   //
   // Set by column *index* through regular-table's own saveColumnSizes /
-  // restoreColumnSizes, not as a plugin_config column_size_override. The override
+  // restoreColumnSizes, not as a plugin_config column_size_override, which maps
-  // path maps the key "__ROW_PATH__" to index tree_header_offset - 1, which with
+  // the single key "__ROW_PATH__" onto one index and so cannot address a
-  // two group_by levels is index 2 — the first data column, not a row label.
+  // multi-level row header at all.
-  // Pinning that was doing nothing visible because that column is the
+  //
-  // grand-total one, which the stylesheet hides.
+  // Note the index arithmetic, which an earlier version of this comment had
+  // wrong: tree_header_offset is group_by.length + 1 and is where the DATA
+  // starts. The row labels are columns 0..group_by.length, of which 0 is the
+  // indent gutter and 1..N are the N levels. tree_header_offset - 1 is the last
+  // *label* column, not a data column; the hidden grand-total column is the
+  // data one at tree_header_offset itself.
   async function fitRowLabels(grid) {
     const cells = [...grid.querySelectorAll('tbody th')]
     if (cells.length === 0) return