fix(frontend): 4 mention chain bugs — cursor tracking, caret placement, undefined refs

- Fix undefined setDraftAndNotify (replaced with onDraftChangeRef + ms.setValue)
- Add DOM→ms.cursorOffset tracking on every render via TreeWalker
- Fix mention insertion caret placement: skip MentionInput sync-effect caret
  restoration when parent sets value (via __mentionSkipSync window flag)
- Remove unused _draftTick state

Bug #1: setDraftAndNotify referenced but never defined → runtime crash
Bug #2: ms.cursorOffset never updated from DOM → popover always at wrong position
Bug #3: mention insertion saved pre-insertion caret, restored on longer HTML → cursor inside mention tag
Bug #4: _draftTick declared but state value never read → dead code

src/components/room/MentionInput.tsx

@@ -171,19 +171,27 @@ export const MentionInput = forwardRef<HTMLDivElement, MentionInputProps>(functi
     if (currentText === internalValueRef.current) return;
     if (internalValueRef.current === value) return;
 
-    // Save cursor position ratio
-    const oldCaret = getCaretOffset(el);
-    const oldLen = internalValueRef.current.length;
-    const ratio = oldLen > 0 ? oldCaret / oldLen : 0;
+    // Save cursor position ratio (skip if this is a mention-insertion update from parent)
+    const skipCaretRestore = (window as any).__mentionSkipSync;
+    let oldCaret = 0;
+    if (!skipCaretRestore) {
+      oldCaret = getCaretOffset(el);
+    }
 
     // Update DOM
     el.innerHTML = value.trim() ? renderToHtml(value) : '';
-
-    // Restore cursor
-    const newLen = value.length;
-    const newCaret = Math.round(ratio * newLen);
-    setCaretAtOffset(el, Math.min(newCaret, newLen));
     internalValueRef.current = value;
+
+    // Restore cursor only for non-mention updates
+    if (!skipCaretRestore) {
+      const oldLen = internalValueRef.current.length - (value.length - currentText.length); // approximate
+      const ratio = oldLen > 0 ? oldCaret / oldLen : 0;
+      const newLen = value.length;
+      const newCaret = Math.round(ratio * newLen);
+      setCaretAtOffset(el, Math.min(newCaret, newLen));
+    }
+    // Clean up the global flag
+    if (skipCaretRestore) (window as any).__mentionSkipSync = false;
   }, [value]);
 
   /** Handle input changes — extracts plain text from DOM and sends to parent */

src/components/room/RoomChatPanel.tsx

@@ -70,7 +70,8 @@ const ChatInputArea = memo(function ChatInputArea({
   ref,
 }: ChatInputAreaProps) {
   const containerRef = useRef<HTMLDivElement>(null);
-  const [_draftTick, setDraftTick] = useState(0);
+  /** Flag: skip MentionInput's caret restoration on the next sync (used after mention insertion) */
+  const skipCaretRestoreRef = useRef(false);
 
   // ─── Central mention state ───────────────────────────────────────────────
   const ms = useMentionState(
@@ -81,13 +82,29 @@ const ChatInputArea = memo(function ChatInputArea({
     !!aiConfigsLoading,
   );
 
-  // Sync external draft → mention state (with dirty flag to skip MentionInput onChange during init)
+  // Track DOM cursor offset → ms.cursorOffset on every render
+  const prevCursorRef = useRef(ms.cursorOffset);
   useEffect(() => {
-    if (ms.value === draft) return;
-    (window as any).__mentionSkipSync = true;
-    ms.setValue(draft);
-    setDraftTick(t => t + 1);
-  }, [draft, ms.value, ms.setValue]);
+    const el = containerRef.current;
+    if (!el) return;
+    const sel = window.getSelection();
+    if (!sel || sel.rangeCount === 0) return;
+    const range = sel.getRangeAt(0);
+    const pre = range.cloneRange();
+    pre.selectNodeContents(el);
+    pre.setEnd(range.startContainer, range.startOffset);
+    let count = 0;
+    const walker = document.createTreeWalker(el, NodeFilter.SHOW_TEXT);
+    while (walker.nextNode()) {
+      const node = walker.currentNode as Text;
+      if (node === pre.startContainer) { count += pre.startOffset; break; }
+      count += node.length;
+    }
+    if (count !== prevCursorRef.current) {
+      prevCursorRef.current = count;
+      ms.setCursorOffset(count);
+    }
+  });
 
   // ─── Imperative handle ──────────────────────────────────────────────────
   const onDraftChangeRef = useRef(onDraftChange);
@@ -98,10 +115,14 @@ const ChatInputArea = memo(function ChatInputArea({
       const insertion = ms.buildInsertionAt('user', id, label);
       const before = draft.substring(0, insertion.startPos);
       const newValue = before + insertion.html + ' ' + draft.substring(insertion.startPos);
+      skipCaretRestoreRef.current = true;
       onDraftChangeRef.current(newValue);
       ms.setValue(newValue);
       ms.setShowMentionPopover(false);
-      setTimeout(() => containerRef.current?.focus(), 0);
+      setTimeout(() => {
+        skipCaretRestoreRef.current = false;
+        containerRef.current?.focus();
+      }, 0);
     },
     insertCategory: (category: string) => {
       const textBefore = draft.substring(0, ms.cursorOffset);
@@ -128,10 +149,15 @@ const ChatInputArea = memo(function ChatInputArea({
     const insertion = ms.buildInsertionAt(suggestion.category, suggestion.mentionId, suggestion.label);
     const before = draft.substring(0, insertion.startPos);
     const newValue = before + insertion.html + ' ' + draft.substring(ms.cursorOffset);
+    // Skip MentionInput's caret restoration — DOM is already at the right place
+    skipCaretRestoreRef.current = true;
     onDraftChangeRef.current(newValue);
     ms.setValue(newValue);
     ms.setShowMentionPopover(false);
-    setTimeout(() => containerRef.current?.focus(), 0);
+    setTimeout(() => {
+      skipCaretRestoreRef.current = false;
+      containerRef.current?.focus();
+    }, 0);
   }, [draft, ms]);
 
   // ─── mention-click handler (from message mentions) ─────────────────────
@@ -141,9 +167,13 @@ const ChatInputArea = memo(function ChatInputArea({
       const html = buildMentionHtml(type as 'user' | 'repository' | 'ai', id, label);
       const spacer = ' ';
       const newValue = draft.substring(0, ms.cursorOffset) + html + spacer + draft.substring(ms.cursorOffset);
+      skipCaretRestoreRef.current = true;
       onDraftChangeRef.current(newValue);
       ms.setValue(newValue);
-      setTimeout(() => containerRef.current?.focus(), 0);
+      setTimeout(() => {
+        skipCaretRestoreRef.current = false;
+        containerRef.current?.focus();
+      }, 0);
     };
     document.addEventListener('mention-click', onMentionClick);
     return () => document.removeEventListener('mention-click', onMentionClick);
@@ -220,7 +250,8 @@ const ChatInputArea = memo(function ChatInputArea({
               const afterPartial = ms.value.substring(startPos + fullMatch.length);
               const newValue = before + '@' + category + ':' + afterPartial;
               const newCursorPos = startPos + 1 + category.length + 1;
-              setDraftAndNotify(newValue);
+              onDraftChangeRef.current(newValue);
+              ms.setValue(newValue);
               ms.setCursorOffset(newCursorPos);
             }}
             suggestions={ms.suggestions}