fix(chat): settle scroll after history load

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

@@ -30,6 +30,8 @@ import styles from "./chat-area.module.css";
 
 const CHAT_AUTO_SCROLL_BOTTOM_THRESHOLD = 96;
 
+type RestoreState = "idle" | "checking" | "restoring" | "settlingBottom" | "done";
+
 export interface ChatAreaProps {
   messages: readonly UiMessage[];
   isReplyingAI: boolean;
@@ -51,9 +53,7 @@ export function ChatArea({
   const scrollRef = useRef<HTMLDivElement>(null);
   const prevLengthRef = useRef(messages.length);
   const restoredScrollRef = useRef(false);
-  const restoreStateRef = useRef<"idle" | "checking" | "restoring" | "done">(
-    "idle",
-  );
+  const restoreStateRef = useRef<RestoreState>("idle");
   const wasNearBottomRef = useRef(true);
 
   const saveCurrentScrollSnapshotNow = (anchorMessageId: string) => {
@@ -68,7 +68,8 @@ export function ChatArea({
       const scrollNode = scrollRef.current;
       const shouldSkipAutoScroll =
         restoreStateRef.current === "checking" ||
-        restoreStateRef.current === "restoring";
+        restoreStateRef.current === "restoring" ||
+        restoreStateRef.current === "settlingBottom";
 
       if (scrollNode && !shouldSkipAutoScroll && wasNearBottomRef.current) {
         scrollNode.scrollTo({
@@ -87,12 +88,7 @@ export function ChatArea({
     if (!scrollNode) return;
 
     let cancelled = false;
-    let frame: number | null = null;
-    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;
+    let stopStableScrollTask: (() => void) | null = null;
 
     restoreStateRef.current = "checking";
 
@@ -102,10 +98,17 @@ export function ChatArea({
 
       if (snapshot === null) {
         restoredScrollRef.current = true;
-        restoreStateRef.current = "done";
-        scrollNode.scrollTo({
-          top: scrollNode.scrollHeight,
-          behavior: "auto",
+        restoreStateRef.current = "settlingBottom";
+        prevLengthRef.current = messages.length;
+        stopStableScrollTask = startStableScrollTask({
+          scrollNode,
+          applyScroll: () => {
+            scrollNode.scrollTop = scrollNode.scrollHeight;
+          },
+          onFinish: () => {
+            restoreStateRef.current = "done";
+            wasNearBottomRef.current = true;
+          },
         });
         return;
       }
@@ -114,40 +117,23 @@ export function ChatArea({
       restoreStateRef.current = "restoring";
       prevLengthRef.current = messages.length;
 
-      const restoreScroll = () => {
-        scrollNode.scrollTop = getChatScrollRestoreTop(scrollNode, snapshot);
-      };
-
-      restoreScroll();
-      frame = window.requestAnimationFrame(restoreScroll);
-      timer = window.setTimeout(restoreScroll, 120);
-      resizeObserver =
-        typeof ResizeObserver === "undefined"
-          ? null
-          : new ResizeObserver(restoreScroll);
-      Array.from(scrollNode.children).forEach((child) => {
-        resizeObserver?.observe(child);
+      stopStableScrollTask = startStableScrollTask({
+        scrollNode,
+        applyScroll: () => {
+          scrollNode.scrollTop = getChatScrollRestoreTop(scrollNode, snapshot);
+        },
+        onFinish: () => {
+          restoreStateRef.current = "done";
+          wasNearBottomRef.current = isNearBottom(scrollNode);
+        },
       });
-      observerTimer = window.setTimeout(() => {
-        resizeObserver?.disconnect();
-      }, 800);
-      lateRestoreTimer = window.setTimeout(restoreScroll, 600);
-      finishRestoreTimer = window.setTimeout(() => {
-        restoreStateRef.current = "done";
-        wasNearBottomRef.current = isNearBottom(scrollNode);
-      }, 850);
     };
 
     void restore();
 
     return () => {
       cancelled = true;
-      if (frame !== null) window.cancelAnimationFrame(frame);
-      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();
+      stopStableScrollTask?.();
     };
   }, [messages.length]);
 
@@ -191,6 +177,47 @@ function isNearBottom(scrollNode: HTMLElement): boolean {
   return distanceFromBottom <= CHAT_AUTO_SCROLL_BOTTOM_THRESHOLD;
 }
 
+function startStableScrollTask({
+  applyScroll,
+  onFinish,
+  scrollNode,
+}: {
+  applyScroll: () => void;
+  onFinish: () => void;
+  scrollNode: HTMLElement;
+}): () => void {
+  let frame: number | null = null;
+  let earlyTimer: number | null = null;
+  let lateTimer: number | null = null;
+  let observerTimer: number | null = null;
+  let finishTimer: number | null = null;
+  const resizeObserver =
+    typeof ResizeObserver === "undefined"
+      ? null
+      : new ResizeObserver(applyScroll);
+
+  applyScroll();
+  frame = window.requestAnimationFrame(applyScroll);
+  earlyTimer = window.setTimeout(applyScroll, 120);
+  lateTimer = window.setTimeout(applyScroll, 600);
+  Array.from(scrollNode.children).forEach((child) => {
+    resizeObserver?.observe(child);
+  });
+  observerTimer = window.setTimeout(() => {
+    resizeObserver?.disconnect();
+  }, 800);
+  finishTimer = window.setTimeout(onFinish, 850);
+
+  return () => {
+    if (frame !== null) window.cancelAnimationFrame(frame);
+    if (earlyTimer !== null) window.clearTimeout(earlyTimer);
+    if (lateTimer !== null) window.clearTimeout(lateTimer);
+    if (observerTimer !== null) window.clearTimeout(observerTimer);
+    if (finishTimer !== null) window.clearTimeout(finishTimer);
+    resizeObserver?.disconnect();
+  };
+}
+
 /** 渲染消息列表（按日期分组插入分隔条） */
 function renderMessagesWithDateHeaders(
   messages: readonly UiMessage[],