[RAPID-fix] 스크롤 sentinel 방식으로 교체: scrollIntoView useLayoutEffect (페인트 전 보장)

frontend/components/messenger/ChatPanel.tsx

@@ -23,7 +23,7 @@ export function ChatPanel({ room }: ChatPanelProps) {
   const { emitTypingStart, emitTypingStop, typingUsers } = useMessengerSocket();
   const scrollRef = useRef<HTMLDivElement>(null);
   const [isAtBottom, setIsAtBottom] = useState(true);
-  const [scrollReady, setScrollReady] = useState(false);
+  const bottomRef = useRef<HTMLDivElement>(null);
   const [isEditingName, setIsEditingName] = useState(false);
   const [editName, setEditName] = useState("");
   const editInputRef = useRef<HTMLInputElement>(null);
@@ -36,22 +36,14 @@ export function ChatPanel({ room }: ChatPanelProps) {
 
   const lastMessageId = messages?.[messages.length - 1]?.id;
 
-  // useLayoutEffect: fires before browser paint → hide + scroll synchronously
+  // Scroll to bottom: sentinel scrollIntoView before paint (no visible jump)
   useLayoutEffect(() => {
-    const el = scrollRef.current;
-    if (!el) return;
-    setScrollReady(false);
-    el.scrollTop = el.scrollHeight;
-    // Reveal after scroll is applied
-    requestAnimationFrame(() => setScrollReady(true));
+    bottomRef.current?.scrollIntoView();
   }, [selectedRoomId, lastMessageId]);
 
-  // Second pass: re-scroll after async images load
+  // Second pass for async image loads
   useEffect(() => {
-    const t = setTimeout(() => {
-      const el = scrollRef.current;
-      if (el) el.scrollTop = el.scrollHeight;
-    }, 600);
+    const t = setTimeout(() => { bottomRef.current?.scrollIntoView(); }, 600);
     return () => clearTimeout(t);
   }, [lastMessageId, selectedRoomId]);
 
@@ -167,7 +159,7 @@ export function ChatPanel({ room }: ChatPanelProps) {
       </div>
 
       {/* Messages */}
-      <div ref={scrollRef} className={`flex-1 min-h-0 overflow-y-auto relative${scrollReady ? "" : " invisible"}`}>
+      <div ref={scrollRef} className="flex-1 min-h-0 overflow-y-auto relative">
         <div className="pt-2">
           {messages?.map((msg, idx) => (
             <div key={msg.id}>
@@ -196,6 +188,7 @@ export function ChatPanel({ room }: ChatPanelProps) {
           <div className="px-4 h-5 flex items-center text-xs text-muted-foreground">
             {roomTyping && roomTyping.length > 0 ? `${roomTyping.join(", ")}님이 입력 중...` : ""}
           </div>
+          <div ref={bottomRef} />
         </div>
       </div>