fix(Table): scope expansion re-measure to changed rows; document RO limits

Address CodeRabbit nitpicks on #6379:

- Watcher: diff the before/after TanStack expanded state and only call
  virtualizer.measureElement on rows whose expanded flag actually
  toggled, instead of sweeping every visible row on every change. When
  the state is the boolean sentinel `true` (expand-all) we fall back
  to the full sweep since there are no individual row ids to diff.
- Watcher: drop the redundant inner `if (!virtualizer)` guard — the
  whole watch is already inside `if (virtualizer) { ... }`.
- JSDoc: note that TanStack's ResizeObserver is attached to the main
  <tr> only, and the built-in expansion watcher fires on expand/collapse
  toggles but not on intra-expansion size changes (late-loading images,
  async content, nested toggles). Users whose expansion region can
  resize after mount should call `virtualizer.measure()` themselves.

Author: claylevering

src/runtime/components/Table.vue

@@ -107,6 +107,13 @@ export interface TableProps<T extends TableData = TableData> extends TableOption
    * `measureElement` already includes the expanded region (e.g. by measuring a wrapper),
    * return only the collapsed-row height to avoid double-counting.
    *
+   * TanStack Virtual's `ResizeObserver` is attached to the main `<tr>` only, and this
+   * component re-measures rows whenever the TanStack expanded state toggles. It does **not**
+   * observe size changes *inside* the expansion sibling — late-loading images, async content,
+   * or nested toggles that resize the expansion region after mount won't trigger an automatic
+   * re-measure. Call `virtualizer.measure()` (or `virtualizer.measureElement(row)`) yourself
+   * in those cases.
+   *
    * Note: when enabled, the divider between rows, sticky and row pinning properties are not supported.
    * @see https://tanstack.com/virtual/latest/docs/api/virtualizer#options
    * @defaultValue false
@@ -472,19 +479,43 @@ function measureRowRef(el: Element | ComponentPublicInstance | null) {
   virtualizer.value.measureElement(el as Element)
 }
 
-// Re-measure rendered rows whenever the expanded state changes. Tanstack's
-// ResizeObserver is attached to the main `<tr>`, but expansion inserts a
-// *sibling* `<tr>` that doesn't resize the main row, so the RO never fires —
-// this manual re-measure keeps the virtualizer's cached heights in sync.
+// Re-measure rows whose expanded state toggled. TanStack's ResizeObserver is
+// attached to the main `<tr>`, but expansion inserts a *sibling* `<tr>` that
+// doesn't resize the main row, so the RO never fires — this manual re-measure
+// keeps the virtualizer's cached heights in sync. Diff the before/after state
+// and only re-measure rows whose expanded flag actually changed. When the
+// expanded state is the sentinel `true` (expand-all) we fall back to a full
+// sweep since there are no individual row ids to diff against.
 if (virtualizer) {
   watch(
     () => tableApi.getState().expanded,
-    () => {
+    (next, prev) => {
       nextTick(() => {
         const root = rootRef.value?.$el as HTMLElement | undefined
-        if (!virtualizer) return
-        root?.querySelectorAll<HTMLElement>('[data-index]').forEach((row) => {
-          virtualizer.value.measureElement(row)
+        if (!root) return
+
+        const measureAll = () => {
+          root.querySelectorAll<HTMLElement>('[data-index]').forEach((row) => {
+            virtualizer.value.measureElement(row)
+          })
+        }
+
+        if (typeof next === 'boolean' || typeof prev === 'boolean') {
+          measureAll()
+          return
+        }
+
+        const prevMap = (prev ?? {}) as Record<string, boolean>
+        const nextMap = (next ?? {}) as Record<string, boolean>
+        const changedIds = new Set<string>()
+        for (const id in nextMap) if (!!nextMap[id] !== !!prevMap[id]) changedIds.add(id)
+        for (const id in prevMap) if (!!nextMap[id] !== !!prevMap[id]) changedIds.add(id)
+
+        changedIds.forEach((id) => {
+          const index = tableApi.getRow(id)?.index
+          if (index === undefined) return
+          const el = root.querySelector<HTMLElement>(`[data-index="${index}"]`)
+          if (el) virtualizer.value.measureElement(el)
         })
       })
     },