fix(markdown-editor): write the table-grip open ref synchronously to close a race

openRef was mirrored from the `open` state through a passive effect, but the
document-level mousemove handler and the stale-target droppers read
openRef.current on continuous events. Between a grip click (setOpen) and the
effect flush, a mousemove saw the stale null and retargeted the grip — and the
about-to-open menu — onto a neighbouring cell. Set the ref synchronously inside
onMenuChange before setOpen and drop the mirroring effect.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/shared/markdown-editor/markdown-editor-table-handles.tsx

@@ -248,10 +248,6 @@ function useTableHandles(editor: Editor): TableHandlesController {
     const openRef = useRef<OpenMenu>(null);
     const cellPosRef = useRef<null | number>(null);
 
-    useEffect(() => {
-        openRef.current = open;
-    }, [open]);
-
     useEffect(() => {
         const dom = editor.view.dom;
 
@@ -367,7 +363,13 @@ function useTableHandles(editor: Editor): TableHandlesController {
     }, [editor]);
 
     const onMenuChange = (menu: 'column' | 'row') => (isOpen: boolean) => {
-        setOpen(isOpen ? menu : null);
+        const next = isOpen ? menu : null;
+
+        // Write the ref synchronously, before setOpen. handleMove/dropStaleTarget are continuous-event handlers
+        // reading openRef.current; mirroring `open` through a passive effect flushes a tick late, so a mousemove
+        // in that window would retarget the grip to a neighbouring cell just as a menu opens on this one.
+        openRef.current = next;
+        setOpen(next);
 
         if (!isOpen) {
             cellPosRef.current = null;