refactor(plugin-gantt): delete the taskListWidth tombstone, keep the one fact it held

.changeset/7421-gantt-tasklistwidth-tombstone.md

@@ -0,0 +1,22 @@
+---
+---
+
+Removes the `taskListWidth_LEGACY_REMOVED` tombstone from `GanttView`, and records the fact
+it was carrying at the site that owns it.
+
+The line was one `const` bound to the literal `null` inside the component body, left behind
+by the finished task-list-width refactor. Measured on `origin/main` `a27d153c2`, the
+identifier appeared exactly once in the whole repo — its own declaration — and zero times in
+a read position; the same probe run returned 20 read sites for the live `taskListWidth` and
+360 for `rowHeight`, so the zero is a reading and not a broken grep. Nothing in the platform
+repo declared it either. It carried no state, no side effect and no contract surface.
+
+What it did carry was a false signal. A name spelling `_LEGACY_REMOVED`, sitting inside the
+component a few hundred lines below the real width derivation, reads as a seam deliberately
+retained for a reason recorded elsewhere, so the next reader goes looking for that reason.
+
+Its trailing comment held one fact worth keeping — that the width now comes from the
+container `useResizeObserver` — and that fact was stated nowhere at the observer call site.
+It moves there rather than dying with the line: the observed container width is named as the
+source the auto-sized row height, base column width and task-list pane width all derive
+from. No behaviour changes, and nothing published moves.

packages/plugin-gantt/src/GanttView.tsx

@@ -872,6 +872,8 @@ export function GanttView({
   const dateLocale = language || undefined;
   const [currentDate, setCurrentDate] = React.useState(() => tzShift.now());
   const containerRef = React.useRef<HTMLDivElement>(null);
+  // Observed container width — the source every auto-sized dimension below
+  // derives from: row height, base column width, and the task-list pane width.
   const { width: containerWidth } = useResizeObserver(containerRef);
   const effectiveWidth = containerWidth || (typeof window !== 'undefined' ? window.innerWidth : 1024);
   const isNarrow = effectiveWidth < 640;
@@ -2320,8 +2322,6 @@ export function GanttView({
       .filter((m) => m.valid);
   }, [markers, timelineRange, dateToX]);
 
-  const taskListWidth_LEGACY_REMOVED = null; // taskListWidth now derived from useResizeObserver above
-
   const headerRef = React.useRef<HTMLDivElement>(null);
   const listRef = React.useRef<HTMLDivElement>(null);
   const timelineRef = React.useRef<HTMLDivElement>(null);