fix(chat): restore image message scroll anchor

src/app/chat/components/chat-area.tsx

@@ -28,6 +28,8 @@ import { LottieMessageBubble } from "./lottie-message-bubble";
 import { MessageBubble } from "./message-bubble";
 import styles from "./chat-area.module.css";
 
+const CHAT_AUTO_SCROLL_BOTTOM_THRESHOLD = 96;
+
 export interface ChatAreaProps {
   messages: readonly UiMessage[];
   isReplyingAI: boolean;
@@ -49,30 +51,31 @@ export function ChatArea({
   const scrollRef = useRef<HTMLDivElement>(null);
   const prevLengthRef = useRef(messages.length);
   const restoredScrollRef = useRef(false);
-  const saveFrameRef = useRef<number | null>(null);
+  const restoreStateRef = useRef<"idle" | "checking" | "restoring" | "done">(
+    "idle",
+  );
+  const wasNearBottomRef = useRef(true);
 
-  const saveCurrentScrollSnapshotNow = () => {
+  const saveCurrentScrollSnapshotNow = (anchorMessageId: string) => {
     const scrollNode = scrollRef.current;
     if (!scrollNode) return;
-    saveChatScrollSnapshot(scrollNode);
-  };
-
-  const saveCurrentScrollSnapshot = () => {
-    if (saveFrameRef.current !== null) return;
-
-    saveFrameRef.current = window.requestAnimationFrame(() => {
-      saveFrameRef.current = null;
-      saveCurrentScrollSnapshotNow();
-    });
+    saveChatScrollSnapshot(scrollNode, { anchorMessageId });
   };
 
   // 新消息 → 滚到底部
   useEffect(() => {
     if (messages.length !== prevLengthRef.current) {
-      scrollRef.current?.scrollTo({
-        top: scrollRef.current.scrollHeight,
-        behavior: "smooth",
-      });
+      const scrollNode = scrollRef.current;
+      const shouldSkipAutoScroll =
+        restoreStateRef.current === "checking" ||
+        restoreStateRef.current === "restoring";
+
+      if (scrollNode && !shouldSkipAutoScroll && wasNearBottomRef.current) {
+        scrollNode.scrollTo({
+          top: scrollNode.scrollHeight,
+          behavior: "smooth",
+        });
+      }
       prevLengthRef.current = messages.length;
     }
   }, [messages.length]);
@@ -88,13 +91,28 @@ export function ChatArea({
     let timer: number | null = null;
     let observerTimer: number | null = null;
     let lateRestoreTimer: number | null = null;
+    let finishRestoreTimer: number | null = null;
     let resizeObserver: ResizeObserver | null = null;
 
+    restoreStateRef.current = "checking";
+
     const restore = async () => {
       const snapshot = await consumeChatScrollSnapshot();
-      if (cancelled || snapshot === null) return;
+      if (cancelled) return;
+
+      if (snapshot === null) {
+        restoredScrollRef.current = true;
+        restoreStateRef.current = "done";
+        scrollNode.scrollTo({
+          top: scrollNode.scrollHeight,
+          behavior: "auto",
+        });
+        return;
+      }
 
       restoredScrollRef.current = true;
+      restoreStateRef.current = "restoring";
+      prevLengthRef.current = messages.length;
 
       const restoreScroll = () => {
         scrollNode.scrollTop = getChatScrollRestoreTop(scrollNode, snapshot);
@@ -114,6 +132,10 @@ export function ChatArea({
         resizeObserver?.disconnect();
       }, 800);
       lateRestoreTimer = window.setTimeout(restoreScroll, 600);
+      finishRestoreTimer = window.setTimeout(() => {
+        restoreStateRef.current = "done";
+        wasNearBottomRef.current = isNearBottom(scrollNode);
+      }, 850);
     };
 
     void restore();
@@ -124,24 +146,18 @@ export function ChatArea({
       if (timer !== null) window.clearTimeout(timer);
       if (observerTimer !== null) window.clearTimeout(observerTimer);
       if (lateRestoreTimer !== null) window.clearTimeout(lateRestoreTimer);
+      if (finishRestoreTimer !== null) window.clearTimeout(finishRestoreTimer);
       resizeObserver?.disconnect();
     };
   }, [messages.length]);
 
   useEffect(() => {
-    const scrollNode = scrollRef.current;
-    const stopListening = listenChatScrollSnapshotSave(
-      saveCurrentScrollSnapshotNow,
-    );
+    const stopListening = listenChatScrollSnapshotSave(({ anchorMessageId }) => {
+      saveCurrentScrollSnapshotNow(anchorMessageId);
+    });
 
     return () => {
       stopListening();
-      if (saveFrameRef.current !== null) {
-        window.cancelAnimationFrame(saveFrameRef.current);
-        saveFrameRef.current = null;
-      }
-      if (!scrollNode) return;
-      saveChatScrollSnapshot(scrollNode);
     };
   }, []);
 
@@ -150,7 +166,9 @@ export function ChatArea({
       ref={scrollRef}
       className={styles.area}
       aria-label="Chat messages"
-      onScroll={saveCurrentScrollSnapshot}
+      onScroll={(event) => {
+        wasNearBottomRef.current = isNearBottom(event.currentTarget);
+      }}
     >
       <AiDisclosureBanner />
 
@@ -167,6 +185,12 @@ export function ChatArea({
   );
 }
 
+function isNearBottom(scrollNode: HTMLElement): boolean {
+  const distanceFromBottom =
+    scrollNode.scrollHeight - scrollNode.scrollTop - scrollNode.clientHeight;
+  return distanceFromBottom <= CHAT_AUTO_SCROLL_BOTTOM_THRESHOLD;
+}
+
 /** 渲染消息列表（按日期分组插入分隔条） */
 function renderMessagesWithDateHeaders(
   messages: readonly UiMessage[],