batch3 T6 review fixes: remove rollback closure, flush-error resync

- remove: capture snapshot inside setPanes functional updater to avoid
  stale-closure rollback under concurrent renders
- flushPendingState: call refresh() on PATCH failure so server truth and
  optimistic local state can't silently diverge
- Drop body.state! non-null assertion via narrowed local

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/usePanes.ts

@@ -19,21 +19,6 @@ export function usePanes(sessionId: string | undefined): {
   const pendingState = useRef<Map<string, PaneState>>(new Map());
   const debounceTimer = useRef<ReturnType<typeof setTimeout> | null>(null);
 
-  const flushPendingState = useCallback(() => {
-    if (debounceTimer.current !== null) {
-      clearTimeout(debounceTimer.current);
-      debounceTimer.current = null;
-    }
-    const snapshot = new Map(pendingState.current);
-    pendingState.current.clear();
-    for (const [id, state] of snapshot) {
-      // fire-and-forget; caller surface handles errors via hook error state
-      void api.panes.update(id, { state }).catch((err) => {
-        setError(err instanceof Error ? err.message : 'pane operation failed');
-      });
-    }
-  }, []);
-
   const refresh = useCallback(async () => {
     if (!sessionId) {
       setPanes(null);
@@ -51,6 +36,23 @@ export function usePanes(sessionId: string | undefined): {
     }
   }, [sessionId]);
 
+  const flushPendingState = useCallback(async () => {
+    if (debounceTimer.current !== null) {
+      clearTimeout(debounceTimer.current);
+      debounceTimer.current = null;
+    }
+    const updates = Array.from(pendingState.current.entries());
+    pendingState.current.clear();
+    if (updates.length === 0) return;
+    try {
+      await Promise.all(updates.map(([id, state]) => api.panes.update(id, { state })));
+    } catch (err) {
+      setError(err instanceof Error ? err.message : 'pane state PATCH failed');
+      // server truth may diverge from optimistic local state; resync
+      void refresh();
+    }
+  }, [refresh]);
+
   // Fetch on mount / sessionId change; preserve previous list while reloading
   // (loading=true but panes stays non-null after first fetch to avoid flash)
   useEffect(() => {
@@ -99,7 +101,8 @@ export function usePanes(sessionId: string | undefined): {
         });
 
         // Coalesce: last state wins within debounce window
-        pendingState.current.set(id, body.state!);
+        const nextState = body.state;
+        pendingState.current.set(id, nextState);
 
         if (debounceTimer.current !== null) {
           clearTimeout(debounceTimer.current);
@@ -124,21 +127,24 @@ export function usePanes(sessionId: string | undefined): {
 
   const remove = useCallback(
     async (id: string): Promise<void> => {
-      // Optimistic remove
-      const previous = panes;
-      setPanes((prev) => (prev ? prev.filter((p) => p.id !== id) : prev));
+      // Optimistic remove — capture snapshot inside functional updater to avoid stale closure
+      let snapshot: Pane[] | null = null;
+      setPanes((prev) => {
+        snapshot = prev;
+        return prev ? prev.filter((p) => p.id !== id) : prev;
+      });
 
       try {
         await api.panes.remove(id);
         await refresh();
       } catch (err) {
-        // Rollback
-        setPanes(previous);
+        // Rollback to the truly-most-recent value captured above
+        setPanes(snapshot);
         setError(err instanceof Error ? err.message : 'pane operation failed');
         throw err;
       }
     },
-    [panes, refresh]
+    [refresh]
   );
 
   return { panes, loading, error, refresh, create, update, remove };