diff --git a/src/renderer/src/ConversationSidebar.tsx b/src/renderer/src/ConversationSidebar.tsx
index ac04536c..0f3234c3 100644
--- a/src/renderer/src/ConversationSidebar.tsx
+++ b/src/renderer/src/ConversationSidebar.tsx
@@ -632,9 +632,13 @@ export const ConversationSidebar = memo(function ConversationSidebar({
     listScope,
     Boolean(conversationActionsId),
   );
+  // Same ids keep the same array: conversations change on every streamed
+  // update, and a new array would rerun the window's layout effect, which
+  // reads scrollTop and forces a synchronous layout mid-stream.
+  const filteredIdsKey = filteredConversations.map((conversation) => conversation.id).join("\n");
   const filteredIds = useMemo(
-    () => filteredConversations.map((conversation) => conversation.id),
-    [filteredConversations],
+    () => (filteredIdsKey ? filteredIdsKey.split("\n") : []),
+    [filteredIdsKey],
   );
   const listWindow = useConversationListWindow({
     ids: filteredIds,
diff --git a/src/renderer/src/OverflowMarquee.test.tsx b/src/renderer/src/OverflowMarquee.test.tsx
index cb556106..a2335803 100644
--- a/src/renderer/src/OverflowMarquee.test.tsx
+++ b/src/renderer/src/OverflowMarquee.test.tsx
@@ -54,6 +54,37 @@ describe('OverflowMarquee', () => {
     ).toBe('')
   })
 
+  it('measures from the resize observer instead of forcing layout on mount', () => {
+    let callback: ResizeObserverCallback | undefined
+    class FakeResizeObserver {
+      constructor(next: ResizeObserverCallback) { callback = next }
+      observe(): void {}
+      disconnect(): void {}
+    }
+    const original = globalThis.ResizeObserver
+    globalThis.ResizeObserver = FakeResizeObserver as unknown as typeof ResizeObserver
+    try {
+      const text = '这是一个明显超过会话列表宽度的完整会话名称'
+      let reads = 0
+      const scrollWidth = Object.getOwnPropertyDescriptor(HTMLElement.prototype, 'scrollWidth')
+      Object.defineProperty(HTMLElement.prototype, 'scrollWidth', { configurable: true, get: () => { reads += 1; return 280 } })
+      try {
+        render(<OverflowMarquee text={text} />)
+        const container = screen.getByTitle(text)
+        setMeasuredWidth(container, 'clientWidth', 120)
+        expect(reads).toBe(0)
+        callback?.([], {} as ResizeObserver)
+        expect(reads).toBe(1)
+        expect(container).toHaveAttribute('data-overflowing', 'true')
+      } finally {
+        if (scrollWidth) Object.defineProperty(HTMLElement.prototype, 'scrollWidth', scrollWidth)
+        else Reflect.deleteProperty(HTMLElement.prototype, 'scrollWidth')
+      }
+    } finally {
+      globalThis.ResizeObserver = original
+    }
+  })
+
   it('keeps one readable text copy while its visual track moves', () => {
     render(<OverflowMarquee text="完整会话名称" />)
 
diff --git a/src/renderer/src/OverflowMarquee.tsx b/src/renderer/src/OverflowMarquee.tsx
index 874a705f..33cc7ae1 100644
--- a/src/renderer/src/OverflowMarquee.tsx
+++ b/src/renderer/src/OverflowMarquee.tsx
@@ -54,7 +54,6 @@ export function OverflowMarquee({
   }, [])
 
   useLayoutEffect(() => {
-    measureOverflow()
     const container = containerRef.current
     const track = trackRef.current
     if (
@@ -62,11 +61,15 @@ export function OverflowMarquee({
       !track ||
       typeof ResizeObserver !== 'function'
     ) {
+      measureOverflow()
       window.addEventListener('resize', measureOverflow)
       return () =>
         window.removeEventListener('resize', measureOverflow)
     }
 
+    // The observer reports both elements once after layout (before paint),
+    // so mounting many rows does not force a synchronous layout per row. A
+    // text change re-creates the observer, whose first report re-measures.
     const observer = new ResizeObserver(measureOverflow)
     observer.observe(container)
     observer.observe(track)