fix(webui): stabilize thread during IME input

webui/src/components/thread/ThreadComposer.tsx

@@ -1799,6 +1799,7 @@ export function ThreadComposer({
   };
 
   const onInput: React.FormEventHandler<HTMLTextAreaElement> = (e) => {
+    if ((e.nativeEvent as InputEvent).isComposing) return;
     const el = e.currentTarget;
     el.style.height = "auto";
     el.style.height = `${Math.min(el.scrollHeight, 260)}px`;

webui/src/components/thread/ThreadViewport.tsx

@@ -186,6 +186,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
   const pendingPromptJumpRef = useRef<string | null>(null);
   const restoreScrollAfterPrependRef =
     useRef<{ height: number; top: number } | null>(null);
+  const composerInputScrollTopRef = useRef<number | null>(null);
   const composerDockHeightRef = useRef(0);
   const [atBottom, setAtBottom] = useState(true);
   const [composerDockHeight, setComposerDockHeight] = useState(0);
@@ -688,9 +689,23 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
             data-testid="thread-composer-dock"
             onInputCapture={(event) => {
               if (event.target instanceof HTMLTextAreaElement) {
+                composerInputScrollTopRef.current = scrollRef.current?.scrollTop ?? null;
                 threadMotionRef.current?.handleComposerInput();
               }
             }}
+            onInput={(event) => {
+              if (!(event.target instanceof HTMLTextAreaElement)) return;
+              const previousScrollTop = composerInputScrollTopRef.current;
+              composerInputScrollTopRef.current = null;
+              const scrollEl = scrollRef.current;
+              if (scrollEl && previousScrollTop !== null) {
+                // Textarea autosizing briefly collapses to `height: auto` while
+                // measuring. Chrome can clamp the sibling thread scrollport in
+                // that intermediate layout; restore it before paint, then let
+                // ResizeObserver handle any real final composer height change.
+                scrollEl.scrollTop = previousScrollTop;
+              }
+            }}
             className={cn(
               "row-start-2 z-10 w-full",
               hasMessages ? "relative bg-background" : "relative self-center",

webui/src/tests/thread-composer.test.tsx

@@ -441,6 +441,33 @@ describe("ThreadComposer", () => {
     expect(input.parentElement?.parentElement?.className).toContain("max-w-[58rem]");
   });
 
+  it("defers textarea autosizing until IME composition commits", () => {
+    render(
+      <ThreadComposer
+        onSend={vi.fn()}
+        placeholder="Type your message..."
+      />,
+    );
+    const input = screen.getByLabelText("Message input") as HTMLTextAreaElement;
+    Object.defineProperty(input, "scrollHeight", {
+      configurable: true,
+      value: 120,
+    });
+    input.style.height = "50px";
+
+    fireEvent.input(input, {
+      target: { value: "zhongwen" },
+      isComposing: true,
+    });
+    expect(input.style.height).toBe("50px");
+
+    fireEvent.input(input, {
+      target: { value: "中文" },
+      isComposing: false,
+    });
+    expect(input.style.height).toBe("120px");
+  });
+
   it("lets long model preset labels use their intrinsic width", () => {
     render(
       <ThreadComposer

webui/src/tests/thread-viewport.test.tsx

@@ -1116,6 +1116,36 @@ describe("ThreadViewport", () => {
     }
   });
 
+  it("restores thread scroll after the textarea autosize measurement collapses it", () => {
+    let scroller: HTMLElement | null = null;
+    const { container } = render(
+      <ThreadViewport
+        messages={messages}
+        isStreaming={false}
+        composer={(
+          <textarea
+            aria-label="Message input"
+            onInput={() => {
+              if (scroller) scroller.scrollTop = 692;
+            }}
+          />
+        )}
+      />,
+    );
+    scroller = getScroller(container);
+    Object.defineProperty(scroller, "scrollTop", {
+      configurable: true,
+      writable: true,
+      value: 700,
+    });
+
+    fireEvent.input(screen.getByLabelText("Message input"), {
+      target: { value: "中文" },
+    });
+
+    expect(scroller.scrollTop).toBe(700);
+  });
+
   it("keeps the thread scrollport above a mobile soft keyboard", async () => {
     const visualViewport = stubVisualViewport({ innerHeight: 800, height: 480 });
     try {