perf(app): avoid stream updates in hidden chats (#7901)

Retained chats already preserve their last visible transcript. Keep their stream-tail selection stable while inactive so incoming chunks do not schedule React work; reveal reads the current model tail.

packages/app/src/agent-stream/use-agent-stream-tail.ts

@@ -0,0 +1,12 @@
+import { useRetainedPanelActive } from "@/components/retained-panel";
+import { useSessionStore } from "@/stores/session-store";
+
+// The view retains its last visible transcript. Selecting a stable value here
+// also prevents incoming chunks from scheduling React work in a hidden panel.
+// Revealing the panel reads the current tail directly from the model.
+export function useAgentStreamTail(serverId: string, agentId: string | undefined) {
+  const active = useRetainedPanelActive();
+  return useSessionStore((state) =>
+    active && agentId ? state.sessions[serverId]?.agentStreamTail.get(agentId) : undefined,
+  );
+}

packages/app/src/panels/agent-panel.tsx

@@ -20,6 +20,7 @@ import { StyleSheet, withUnistyles } from "react-native-unistyles";
 import invariant from "tiny-invariant";
 import { shallow, useShallow } from "zustand/shallow";
 import { useStoreWithEqualityFn } from "zustand/traditional";
+import { useAgentStreamTail } from "@/agent-stream/use-agent-stream-tail";
 import { AgentStreamView, type AgentStreamViewHandle } from "@/agent-stream/view";
 import { ArchivedAgentCallout } from "@/components/archived-agent-callout";
 import { ComposerDock } from "@/composer/dock";
@@ -1365,9 +1366,7 @@ const AgentStreamSection = memo(function AgentStreamSection({
   const bottomOverlayControlClearance = hasVisibleComposerTracks
     ? resolveComposerTrackControlClearance(isCompactFormFactor)
     : 0;
-  const streamItemsRaw = useSessionStore((state) =>
-    agentId ? state.sessions[serverId]?.agentStreamTail?.get(agentId) : undefined,
-  );
+  const streamItemsRaw = useAgentStreamTail(serverId, agentId);
   const pendingMessageSubmissions = useSessionStore(
     useShallow((state) =>
       agentId

packages/app/src/panels/retained-chat-content.browser.test.tsx

@@ -1,11 +1,15 @@
 import React, {
   act,
+  Profiler,
   useCallback,
   useEffect,
   useLayoutEffect,
   useState,
   useSyncExternalStore,
 } from "react";
+import { useAgentStreamTail } from "@/agent-stream/use-agent-stream-tail";
+import { useSessionStore } from "@/stores/session-store";
+import type { StreamItem } from "@/types/stream";
 import { createRoot, type Root } from "react-dom/client";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { RetainedPanelActivity, useRetainedPanelActive } from "@/components/retained-panel";
@@ -220,3 +224,69 @@ describe("workspace sheet environment", () => {
     source.unmount();
   });
 });
+
+for (const [runtime, Chat] of [
+  ["native", NativeChat],
+  ["web", WebChat],
+] as const) {
+  it(`${runtime} ignores hidden stream chunks and reads the latest transcript on reveal`, async () => {
+    const serverId = `hidden-stream-${runtime}`;
+    const agentId = "background-agent";
+    const store = useSessionStore.getState();
+    store.initializeSession(serverId, null);
+    const profile = {
+      commits: 0,
+      onRender() {
+        profile.commits++;
+      },
+    };
+    const publish = (text: string) => {
+      const item: StreamItem = {
+        id: "message",
+        kind: "assistant_message",
+        text,
+        timestamp: new Date(),
+      };
+      store.setAgentStreamTail(serverId, new Map([[agentId, [item]]]));
+    };
+    function Content() {
+      const items = useAgentStreamTail(serverId, agentId);
+      const [draft] = useState("unsent draft");
+      return (
+        <span>
+          {draft}:{items?.[0]?.kind === "assistant_message" ? items[0].text : ""}
+        </span>
+      );
+    }
+    const show = (active: boolean) =>
+      act(() =>
+        root.render(
+          <Profiler id="workspace" onRender={profile.onRender}>
+            <RetainedPanelActivity active={active}>
+              <Chat>
+                <Content />
+              </Chat>
+            </RetainedPanelActivity>
+          </Profiler>,
+        ),
+      );
+    try {
+      publish("first chunk");
+      show(true);
+      expect(container.textContent).toBe("unsent draft:first chunk");
+      show(false);
+      const hiddenCommits = profile.commits;
+      for (let index = 0; index < 10; index++) {
+        await act(async () => publish(`chunk ${index}`));
+      }
+      expect(profile.commits).toBe(hiddenCommits);
+      show(true);
+      expect(container.textContent).toBe("unsent draft:chunk 9");
+      await act(async () => publish("live after reveal"));
+      expect(container.textContent).toBe("unsent draft:live after reveal");
+    } finally {
+      act(() => root.render(null));
+      store.clearSession(serverId);
+    }
+  });
+}