fix(frontend): block selectionchange during MentionInput DOM updates

Root cause: when MentionInput updates innerHTML to reflect value changes,
the browser resets the selection to position 0. The selectionchange event
handler reads this wrong cursor position and sets ms.cursorOffset=0,
breaking mentionState calculation for the popover.

Fix:
- MentionInput sets window.__mentionBlockSelection=true before innerHTML
- Clears it via requestAnimationFrame after caret is restored
- selectionchange handler skips cursor reading when flag is set

src/components/room/MentionInput.tsx

@@ -166,7 +166,6 @@ export const MentionInput = forwardRef<HTMLDivElement, MentionInputProps>(functi
     if (!el) return;
 
     // If the DOM already contains what we want, nothing to do.
-    // (Programmatic updates via onChange already set the DOM correctly.)
     if (getPlainText(el) === value) {
       isUserInputRef.current = false;
       return;
@@ -176,6 +175,9 @@ export const MentionInput = forwardRef<HTMLDivElement, MentionInputProps>(functi
     const oldCaret = isUserInputRef.current ? getCaretOffset(el) : 0;
     const oldLen = internalValueRef.current.length;
 
+    // Block selectionchange handler from reading wrong cursor position during DOM update
+    (window as any).__mentionBlockSelection = true;
+
     // Update DOM
     el.innerHTML = value.trim() ? renderToHtml(value) : '';
     internalValueRef.current = value;
@@ -187,6 +189,11 @@ export const MentionInput = forwardRef<HTMLDivElement, MentionInputProps>(functi
       const newLen = value.length;
       setCaretAtOffset(el, Math.round(ratio * newLen));
     }
+
+    // Unblock selectionchange after DOM has settled
+    requestAnimationFrame(() => {
+      (window as any).__mentionBlockSelection = false;
+    });
   }, [value]);
 
   /** Handle input changes — extracts plain text from DOM and sends to parent */

src/components/room/RoomChatPanel.tsx

@@ -83,12 +83,13 @@ const ChatInputArea = memo(function ChatInputArea({
 
   // ─── Track DOM cursor offset → ms.cursorOffset on user navigation ──────
   // Uses selectionchange event which fires when caret moves via arrows/clicks.
-  // Does NOT fire during programmatic updates, avoiding cursor jumps.
+  // Ignores programmatic DOM updates (blocked via window.__mentionBlockSelection).
   const prevCursorRef = useRef(ms.cursorOffset);
   const skipCursorTrackingRef = useRef(false);
   useEffect(() => {
     const readCursor = () => {
       if (skipCursorTrackingRef.current) return;
+      if ((window as any).__mentionBlockSelection) return;
       const el = containerRef.current;
       if (!el) return;
       const sel = window.getSelection();