fix(ui): anchor issue composer during live updates

ui/src/components/IssueChatThread.test.tsx

@@ -19,6 +19,16 @@ const { threadMessagesMock } = vi.hoisted(() => ({
   threadMessagesMock: vi.fn(() => <div data-testid="thread-messages" />),
 }));
 
+const {
+  captureComposerViewportSnapshotMock,
+  restoreComposerViewportSnapshotMock,
+  shouldPreserveComposerViewportMock,
+} = vi.hoisted(() => ({
+  captureComposerViewportSnapshotMock: vi.fn(),
+  restoreComposerViewportSnapshotMock: vi.fn(),
+  shouldPreserveComposerViewportMock: vi.fn(),
+}));
+
 vi.mock("@assistant-ui/react", () => ({
   AssistantRuntimeProvider: ({ children }: { children: ReactNode }) => <div>{children}</div>,
   ThreadPrimitive: {
@@ -55,6 +65,16 @@ vi.mock("./transcript/useLiveRunTranscripts", () => ({
   }),
 }));
 
+vi.mock("../lib/issue-chat-scroll", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("../lib/issue-chat-scroll")>();
+  return {
+    ...actual,
+    captureComposerViewportSnapshot: captureComposerViewportSnapshotMock.mockImplementation(actual.captureComposerViewportSnapshot),
+    restoreComposerViewportSnapshot: restoreComposerViewportSnapshotMock.mockImplementation(actual.restoreComposerViewportSnapshot),
+    shouldPreserveComposerViewport: shouldPreserveComposerViewportMock.mockImplementation(actual.shouldPreserveComposerViewport),
+  };
+});
+
 vi.mock("./MarkdownBody", () => ({
   MarkdownBody: ({ children }: { children: ReactNode }) => <div>{children}</div>,
 }));
@@ -133,6 +153,9 @@ describe("IssueChatThread", () => {
     appendMock.mockReset();
     markdownEditorFocusMock.mockReset();
     threadMessagesMock.mockReset();
+    captureComposerViewportSnapshotMock.mockClear();
+    restoreComposerViewportSnapshotMock.mockClear();
+    shouldPreserveComposerViewportMock.mockClear();
   });
 
   it("drops the count heading and does not use an internal scrollbox", () => {
@@ -495,6 +518,62 @@ describe("IssueChatThread", () => {
     });
   });
 
+  it("requests composer viewport restoration when live messages arrive while the composer is visible", () => {
+    const root = createRoot(container);
+    const scrollByMock = vi.spyOn(window, "scrollBy").mockImplementation(() => {});
+    shouldPreserveComposerViewportMock.mockReturnValue(true);
+    captureComposerViewportSnapshotMock.mockReturnValue({ composerViewportTop: 420 });
+
+    act(() => {
+      root.render(
+        <MemoryRouter>
+          <IssueChatThread
+            comments={[]}
+            linkedRuns={[]}
+            timelineEvents={[]}
+            liveRuns={[]}
+            onAdd={async () => {}}
+            enableLiveTranscriptPolling={false}
+          />
+        </MemoryRouter>,
+      );
+    });
+
+    act(() => {
+      root.render(
+        <MemoryRouter>
+          <IssueChatThread
+            comments={[]}
+            linkedRuns={[]}
+            timelineEvents={[]}
+            liveRuns={[{
+              id: "run-1",
+              issueId: "issue-1",
+              status: "running",
+              invocationSource: "comment",
+              triggerDetail: null,
+              startedAt: "2026-04-06T12:00:00.000Z",
+              finishedAt: null,
+              createdAt: "2026-04-06T12:00:00.000Z",
+              agentId: "agent-1",
+              agentName: "Agent 1",
+              adapterType: "codex_local",
+            }]}
+            onAdd={async () => {}}
+            enableLiveTranscriptPolling={false}
+          />
+        </MemoryRouter>,
+      );
+    });
+
+    expect(restoreComposerViewportSnapshotMock).toHaveBeenCalled();
+
+    scrollByMock.mockRestore();
+    act(() => {
+      root.unmount();
+    });
+  });
+
   it("folds chain-of-thought when the same message transitions from running to complete", () => {
     expect(resolveAssistantMessageFoldedState({
       messageId: "message-1",

ui/src/components/IssueChatThread.tsx

@@ -15,6 +15,7 @@ import {
   useContext,
   useEffect,
   useImperativeHandle,
+  useLayoutEffect,
   useMemo,
   useRef,
   useState,
@@ -70,6 +71,7 @@ import { restoreSubmittedCommentDraft } from "../lib/comment-submit-draft";
 import {
   captureComposerViewportSnapshot,
   restoreComposerViewportSnapshot,
+  shouldPreserveComposerViewport,
 } from "../lib/issue-chat-scroll";
 import { formatAssigneeUserLabel } from "../lib/assignees";
 import { timeAgo } from "../lib/timeAgo";
@@ -1884,6 +1886,9 @@ export function IssueChatThread({
   const location = useLocation();
   const hasScrolledRef = useRef(false);
   const bottomAnchorRef = useRef<HTMLDivElement | null>(null);
+  const composerViewportAnchorRef = useRef<HTMLDivElement | null>(null);
+  const composerViewportSnapshotRef = useRef<ReturnType<typeof captureComposerViewportSnapshot>>(null);
+  const preserveComposerViewportRef = useRef(false);
   const displayLiveRuns = useMemo(() => {
     const deduped = new Map<string, LiveRunForIssue>();
     for (const run of liveRuns) {
@@ -1988,6 +1993,19 @@ export function IssueChatThread({
     onCancel: onCancelRun,
   });
 
+  useLayoutEffect(() => {
+    const composerElement = composerViewportAnchorRef.current;
+    if (preserveComposerViewportRef.current) {
+      restoreComposerViewportSnapshot(
+        composerViewportSnapshotRef.current,
+        composerElement,
+      );
+    }
+
+    composerViewportSnapshotRef.current = captureComposerViewportSnapshot(composerElement);
+    preserveComposerViewportRef.current = shouldPreserveComposerViewport(composerElement);
+  }, [messages]);
+
   useEffect(() => {
     const hash = location.hash;
     if (!(hash.startsWith("#comment-") || hash.startsWith("#activity-") || hash.startsWith("#run-"))) return;
@@ -2096,20 +2114,22 @@ export function IssueChatThread({
         </IssueChatErrorBoundary>
 
         {showComposer ? (
-          <IssueChatComposer
-            ref={composerRef}
-            onImageUpload={imageUploadHandler}
-            onAttachImage={onAttachImage}
-            draftKey={draftKey}
-            enableReassign={enableReassign}
-            reassignOptions={reassignOptions}
-            currentAssigneeValue={currentAssigneeValue}
-            suggestedAssigneeValue={suggestedAssigneeValue}
-            mentions={mentions}
-            agentMap={agentMap}
-            composerDisabledReason={composerDisabledReason}
-            issueStatus={issueStatus}
-          />
+          <div ref={composerViewportAnchorRef}>
+            <IssueChatComposer
+              ref={composerRef}
+              onImageUpload={imageUploadHandler}
+              onAttachImage={onAttachImage}
+              draftKey={draftKey}
+              enableReassign={enableReassign}
+              reassignOptions={reassignOptions}
+              currentAssigneeValue={currentAssigneeValue}
+              suggestedAssigneeValue={suggestedAssigneeValue}
+              mentions={mentions}
+              agentMap={agentMap}
+              composerDisabledReason={composerDisabledReason}
+              issueStatus={issueStatus}
+            />
+          </div>
         ) : null}
       </div>
       </IssueChatCtx.Provider>

ui/src/lib/issue-chat-scroll.test.ts

@@ -4,6 +4,7 @@ import { describe, expect, it, vi } from "vitest";
 import {
   captureComposerViewportSnapshot,
   restoreComposerViewportSnapshot,
+  shouldPreserveComposerViewport,
 } from "./issue-chat-scroll";
 
 function mockTop(element: HTMLElement, top: number) {
@@ -70,4 +71,28 @@ describe("issue-chat-scroll", () => {
     composer.remove();
     mainContent.remove();
   });
+
+  it("preserves the composer viewport when the composer is visible", () => {
+    const composer = document.createElement("div");
+    document.body.appendChild(composer);
+    mockTop(composer, 540);
+
+    expect(shouldPreserveComposerViewport(composer)).toBe(true);
+
+    composer.remove();
+  });
+
+  it("preserves the composer viewport when focus stays inside the composer", () => {
+    const composer = document.createElement("div");
+    const input = document.createElement("textarea");
+    composer.appendChild(input);
+    document.body.appendChild(composer);
+    mockTop(composer, 1200);
+
+    input.focus();
+
+    expect(shouldPreserveComposerViewport(composer)).toBe(true);
+
+    composer.remove();
+  });
 });

ui/src/lib/issue-chat-scroll.ts

@@ -36,6 +36,22 @@ export function captureComposerViewportSnapshot(
   };
 }
 
+export function shouldPreserveComposerViewport(
+  composerElement: HTMLElement | null,
+  doc: Document = document,
+) {
+  if (!composerElement) return false;
+
+  const activeElement = doc.activeElement;
+  if (activeElement instanceof Node && composerElement.contains(activeElement)) {
+    return true;
+  }
+
+  const viewportHeight = doc.defaultView?.innerHeight ?? window.innerHeight;
+  const rect = composerElement.getBoundingClientRect();
+  return rect.bottom > 0 && rect.top < viewportHeight;
+}
+
 export function restoreComposerViewportSnapshot(
   snapshot: ComposerViewportSnapshot | null,
   composerElement: HTMLElement | null,