fix(chat): 修复内容大幅增长时滚动锁定不及时的问题

- 调整 vue-stick-to-bottom resize spring 参数，加快动画响应
  (damping 0.68→0.5, stiffness 0.11→0.28, mass 1.05→0.75)
- 在 scrollListener 中添加高度突变检测，当 scrollHeight 大幅增长
  （超过 200px 或 viewport 35%）且用户未脱离锁定时，用 instant
  瞬间追底，绕过 spring 动画延迟和去重机制

修复场景：思考块展开、Markdown 表格渲染等造成大幅度垂直距离增加时
滚动锁定失效或锁住不及时的问题

static/src/components/chat/ChatArea.vue

@@ -790,9 +790,9 @@ const {
   stopScroll
 } = useStickToBottom({
   resize: {
-    damping: 0.68,
-    stiffness: 0.11,
-    mass: 1.05
+    damping: 0.5,
+    stiffness: 0.28,
+    mass: 0.75
   },
   initial: 'instant'
 });
@@ -809,6 +809,7 @@ let lastUserWheelUpTs = 0;
 let lastProgrammaticHintTs = 0;
 let lastProgrammaticHintSource = '';
 let suppressUserIntentUntil = 0;
+let isHandlingLargeGrowth = false;
 let scrollListener: ((event: Event) => void) | null = null;
 let wheelListener: ((event: WheelEvent) => void) | null = null;
 let traceAttachLogged = false;
@@ -924,6 +925,32 @@ function attachBounceListener() {
     const heightDelta = height - lastObservedHeight;
     lastObservedTop = top;
     lastObservedHeight = height;
+
+    // 大幅高度增长检测：当 scrollHeight 突增（内容大幅展开/渲染），
+    // 且用户没有主动脱离锁定（escapedFromLock=false）时，
+    // 用 instant 瞬间追底，绕过 spring 动画的延迟。
+    // 这解决了思考块展开、Markdown 表格渲染等场景下滚动跟不上内容增长的问题。
+    const largeGrowthThreshold = Math.max(200, target.clientHeight * 0.35 || 200);
+    if (
+      !isHandlingLargeGrowth &&
+      heightDelta > largeGrowthThreshold &&
+      !escapedFromLock.value
+    ) {
+      isHandlingLargeGrowth = true;
+      markProgrammaticHint('ChatArea.largeGrowth');
+      suppressUserIntentUntil = Date.now() + 900;
+      scrollToBottom({ animation: 'instant', preserveScrollPosition: false });
+      bounceTraceLog(
+        'large-growth:instant-scroll',
+        { heightDelta, threshold: largeGrowthThreshold, ...getScrollMetrics(target) },
+        'large-growth:instant-scroll',
+        120
+      );
+      setTimeout(() => {
+        isHandlingLargeGrowth = false;
+      }, 400);
+    }
+
     const now = Date.now();
     const trusted = (event as any).isTrusted === true;
     const closeToProgrammaticHint = now - lastProgrammaticHintTs <= 140;