fix(web): 回到顶部改用 rAF 自驱动动画，修复消息多时平滑滚动被 virtualizer 纠正写入打断停在半路

web/src/components/Chat/MessageList.tsx

@@ -27,6 +27,7 @@ export function MessageList({
   const containerRef = useRef<HTMLDivElement>(null);
   const isAtBottomRef = useRef(true);
   const prevShowBottomRef = useRef(false);
+  const scrollTopRafRef = useRef(0);
   const prevViewKeyRef = useRef(viewKey);
   const viewKeyRef = useRef(viewKey);
   viewKeyRef.current = viewKey;
@@ -65,8 +66,16 @@ export function MessageList({
 
   // ---- scroll helpers ----
 
+  const stopScrollTopAnimation = useCallback(() => {
+    if (scrollTopRafRef.current !== 0) {
+      cancelAnimationFrame(scrollTopRafRef.current);
+      scrollTopRafRef.current = 0;
+    }
+  }, []);
+
   const scrollToBottom = useCallback(
     (behavior: ScrollBehavior = 'smooth') => {
+      stopScrollTopAnimation();
       isAtBottomRef.current = true;
       setShowScrollToBottom(false);
       setNewMessageCount(0);
@@ -74,12 +83,30 @@ export function MessageList({
         virtualizer.scrollToIndex(messages.length - 1, { align: 'end', behavior });
       }
     },
-    [virtualizer, messages.length],
+    [virtualizer, messages.length, stopScrollTopAnimation],
   );
 
+  // 长距离滚回顶部不能用原生 behavior:'smooth'：滚动途中上方行首次被测量时，
+  // virtualizer 会写 scrollTop 做偏移纠正（resizeItem → applyScrollAdjustment），
+  // 程序化 scrollTop 写入会直接打断浏览器平滑滚动动画，消息越多越容易停在半路。
+  // 改为 rAF 自驱动动画，每帧覆写 scrollTop，纠正写入下一帧即被覆盖，必达顶部。
   const scrollToTop = useCallback(() => {
-    containerRef.current?.scrollTo({ top: 0, behavior: 'smooth' });
-  }, []);
+    const el = containerRef.current;
+    if (!el) return;
+    stopScrollTopAnimation();
+    const from = el.scrollTop;
+    if (from <= 0) return;
+    const duration = Math.min(800, 250 + from / 8);
+    const start = performance.now();
+    const easeInOutCubic = (t: number) =>
+      t < 0.5 ? 4 * t * t * t : 1 - Math.pow(-2 * t + 2, 3) / 2;
+    const step = (now: number) => {
+      const p = Math.min(1, (now - start) / duration);
+      el.scrollTop = from * (1 - easeInOutCubic(p));
+      scrollTopRafRef.current = p < 1 ? requestAnimationFrame(step) : 0;
+    };
+    scrollTopRafRef.current = requestAnimationFrame(step);
+  }, [stopScrollTopAnimation]);
 
   // ---- scroll event: track whether user is at bottom ----
 
@@ -126,6 +153,7 @@ export function MessageList({
 
     if (viewChanged) {
       // View switched (e.g. breadcrumb navigation): restore saved scroll position
+      stopScrollTopAnimation();
       prevMessageCountRef.current = messages.length;
       const key = viewKey ?? '';
       const savedPos = scrollPositionsRef.current.get(key);
@@ -148,6 +176,7 @@ export function MessageList({
     prevMessageCountRef.current = messages.length;
 
     if (lastMessage.role === 'user' || isAtBottomRef.current) {
+      stopScrollTopAnimation();
       virtualizer.scrollToIndex(messages.length - 1, { align: 'end', behavior: 'instant' });
       // 用户自己发消息或已在底部时，不需要计数
       if (newCount > 0) {
@@ -157,7 +186,7 @@ export function MessageList({
       // 只有真正新增了消息条数时才累加（流式 delta 不增加条数，不计数）
       setNewMessageCount((prev) => prev + newCount);
     }
-  }, [messages, viewKey, virtualizer]);
+  }, [messages, viewKey, virtualizer, stopScrollTopAnimation]);
 
   // ---- mount: always scroll to bottom if messages already loaded ----
 
@@ -175,6 +204,7 @@ export function MessageList({
     const idx = messageIdToIndex.get(highlightedMessageId);
     if (idx === undefined) return;
 
+    stopScrollTopAnimation();
     virtualizer.scrollToIndex(idx, { align: 'center', behavior: 'smooth' });
 
     // 高亮 class 需等 DOM 渲染后操作
@@ -188,7 +218,25 @@ export function MessageList({
         targetElement.classList.remove('todo-highlight');
       }, 2000);
     });
-  }, [highlightedMessageId, messageIdToIndex, virtualizer]);
+  }, [highlightedMessageId, messageIdToIndex, virtualizer, stopScrollTopAnimation]);
+
+  // ---- 滚顶动画生命周期 ----
+
+  // 用户手动滚动（滚轮/触摸）时打断滚顶动画，交还滚动控制权
+  const hasMessages = messages.length > 0;
+  useEffect(() => {
+    const el = containerRef.current;
+    if (!el) return;
+    el.addEventListener('wheel', stopScrollTopAnimation, { passive: true });
+    el.addEventListener('touchstart', stopScrollTopAnimation, { passive: true });
+    return () => {
+      el.removeEventListener('wheel', stopScrollTopAnimation);
+      el.removeEventListener('touchstart', stopScrollTopAnimation);
+    };
+  }, [hasMessages, stopScrollTopAnimation]);
+
+  // 卸载时取消未完成的动画
+  useEffect(() => stopScrollTopAnimation, [stopScrollTopAnimation]);
 
   // ---- 行高强制重测（修复 tanstack virtual-core 3.17.x 陈旧高度导致行重叠）----
   // 3.17.x 在滚动状态会跳过同步测量、对缓冲区外的行跳过 RO 更新并复用缓存高度；