Stop issue page jumps during live run updates

ui/src/components/IssueChatThread.test.tsx

@@ -518,7 +518,59 @@ describe("IssueChatThread", () => {
     });
   });
 
-  it("requests composer viewport restoration when live messages arrive while the composer is visible", () => {
+  it("does not restore the composer viewport for passive live updates by default", () => {
+    const root = createRoot(container);
+
+    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).not.toHaveBeenCalled();
+
+    act(() => {
+      root.unmount();
+    });
+  });
+
+  it("requests composer viewport restoration when live messages arrive during active composer interaction", () => {
     const root = createRoot(container);
     const scrollByMock = vi.spyOn(window, "scrollBy").mockImplementation(() => {});
     shouldPreserveComposerViewportMock.mockReturnValue(true);

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

@@ -72,12 +72,12 @@ describe("issue-chat-scroll", () => {
     mainContent.remove();
   });
 
-  it("preserves the composer viewport when the composer is visible", () => {
+  it("does not preserve the composer viewport just because the composer is visible", () => {
     const composer = document.createElement("div");
     document.body.appendChild(composer);
     mockTop(composer, 540);
 
-    expect(shouldPreserveComposerViewport(composer)).toBe(true);
+    expect(shouldPreserveComposerViewport(composer)).toBe(false);
 
     composer.remove();
   });

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

@@ -46,10 +46,7 @@ export function shouldPreserveComposerViewport(
   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;
+  return false;
 }
 
 export function restoreComposerViewportSnapshot(