[RAPID-fix] 채팅방 스크롤 점프 제거: useEffect → useLayoutEffect (페인트 전 처리)

frontend/components/messenger/ChatPanel.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import React, { useEffect, useRef, useState } from "react";
+import React, { useEffect, useLayoutEffect, useRef, useState } from "react";
 import { MessageSquare, Pencil, Check, X, ChevronsDown } from "lucide-react";
 import { useMessages, useMarkAsRead, useUpdateRoom } from "@/hooks/useMessenger";
 import { useAuth } from "@/hooks/useAuth";
@@ -36,21 +36,18 @@ export function ChatPanel({ room }: ChatPanelProps) {
 
   const lastMessageId = messages?.[messages.length - 1]?.id;
 
-  // Hide messages until scrolled to bottom (prevents visible jump on room open)
+  // useLayoutEffect: fires before browser paint → hide + scroll synchronously
-  useEffect(() => {
+  useLayoutEffect(() => {
+    const el = scrollRef.current;
+    if (!el) return;
     setScrollReady(false);
-  }, [selectedRoomId]);
+    el.scrollTop = el.scrollHeight;
+    // Reveal after scroll is applied
+    requestAnimationFrame(() => setScrollReady(true));
+  }, [selectedRoomId, lastMessageId]);
 
-  // Scroll to bottom when messages load or room changes
+  // Second pass: re-scroll after async images load
-  // Two-pass: immediate rAF + 600ms delayed (for async image loads)
   useEffect(() => {
-    requestAnimationFrame(() => {
-      const el = scrollRef.current;
-      if (el) {
-        el.scrollTop = el.scrollHeight;
-        setScrollReady(true);
-      }
-    });
     const t = setTimeout(() => {
       const el = scrollRef.current;
       if (el) el.scrollTop = el.scrollHeight;