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

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

[RAPID-fix] 스크롤 근본 원인 수정: isOpen deps 추가로 메신저 열릴 때마다 하단 스크롤

[RAPID-fix] 스크롤 ResizeObserver 추가: 이미지 로드 후 높이 변화 감지해 자동 하단 스크롤

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";
@@ -16,14 +16,14 @@ interface ChatPanelProps {
 
 export function ChatPanel({ room }: ChatPanelProps) {
   const { user } = useAuth();
-  const { selectedRoomId } = useMessengerContext();
+  const { selectedRoomId, isOpen } = useMessengerContext();
   const { data: messages } = useMessages(selectedRoomId);
   const markAsRead = useMarkAsRead();
   const updateRoom = useUpdateRoom();
   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);
@@ -36,34 +36,41 @@ export function ChatPanel({ room }: ChatPanelProps) {
 
   const lastMessageId = messages?.[messages.length - 1]?.id;
 
-  // Hide messages until scrolled to bottom (prevents visible jump on room open)
-  useEffect(() => {
-    setScrollReady(false);
-  }, [selectedRoomId]);
+  // Scroll to bottom: sentinel scrollIntoView before paint (no visible jump)
+  // Scroll to bottom on room open / new message
+  useLayoutEffect(() => {
+    if (isOpen) bottomRef.current?.scrollIntoView();
+  }, [selectedRoomId, lastMessageId, isOpen]);
 
-  // Scroll to bottom when messages load or room changes
-  // Two-pass: immediate rAF + 600ms delayed (for async image loads)
+  // ResizeObserver: re-scroll whenever content height changes (images loading, etc.)
+  const shouldAutoScrollRef = useRef(true);
   useEffect(() => {
-    requestAnimationFrame(() => {
-      const el = scrollRef.current;
-      if (el) {
-        el.scrollTop = el.scrollHeight;
-        setScrollReady(true);
+    const el = scrollRef.current;
+    if (!el) return;
+    const inner = el.firstElementChild as HTMLElement | null;
+    if (!inner) return;
+    const ro = new ResizeObserver(() => {
+      if (shouldAutoScrollRef.current) {
+        bottomRef.current?.scrollIntoView();
       }
     });
-    const t = setTimeout(() => {
-      const el = scrollRef.current;
-      if (el) el.scrollTop = el.scrollHeight;
-    }, 600);
-    return () => clearTimeout(t);
-  }, [lastMessageId, selectedRoomId]);
+    ro.observe(inner);
+    return () => ro.disconnect();
+  }, [selectedRoomId]);
+
+  // Track whether user has scrolled up (disable auto-scroll while reading old messages)
+  useEffect(() => {
+    shouldAutoScrollRef.current = true;
+  }, [selectedRoomId, lastMessageId]);
 
   // Re-attach scroll listener whenever room changes (scrollRef mounts after room is set)
   useEffect(() => {
     const el = scrollRef.current;
     if (!el) return;
     const onScroll = () => {
-      setIsAtBottom(el.scrollHeight - el.scrollTop - el.clientHeight < 60);
+      const atBottom = el.scrollHeight - el.scrollTop - el.clientHeight < 60;
+      setIsAtBottom(atBottom);
+      shouldAutoScrollRef.current = atBottom;
     };
     el.addEventListener("scroll", onScroll);
     return () => el.removeEventListener("scroll", onScroll);
@@ -170,7 +177,7 @@ export function ChatPanel({ room }: ChatPanelProps) {
       </div>
 
       {/* Messages */}
-      <div ref={scrollRef} className={`flex-1 min-h-0 overflow-y-auto relative${scrollReady ? "" : " invisible"}`}>
+      <div ref={scrollRef} className="flex-1 min-h-0 overflow-y-auto relative">
         <div className="pt-2">
           {messages?.map((msg, idx) => (
             <div key={msg.id}>
@@ -199,6 +206,7 @@ export function ChatPanel({ room }: ChatPanelProps) {
           <div className="px-4 h-5 flex items-center text-xs text-muted-foreground">
             {roomTyping && roomTyping.length > 0 ? `${roomTyping.join(", ")}님이 입력 중...` : ""}
           </div>
+          <div ref={bottomRef} />
         </div>
       </div>