fix(webui): stabilize thread during IME input

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",