Session Info scoped to a subagent holds still while the reader scrolls

webviews/src/agent-session/acpmux/App.tsx

@@ -8140,19 +8140,23 @@ function AcpmuxPane() {
       ].join("\u0000")
     : "";
   const scopedRows = shownSubagent === undefined ? undefined : snapshot.subagentViews?.[shownSubagent];
+  // Session Info lists the subagent's rows as they stood when the reader started scrolling, until
+  // the scroll settles, as the chat's card does with `cardRows` (summary/heldWhileScrolling.ts,
+  // cmux#29639). Another subagent, or another chat, takes its rows at once.
+  const cardScopedRows = useHeldWhileScrolling(scopedRows, `${shownSessionId ?? ""}\u0000${shownSubagent ?? ""}`);
   const sessionInfoSubagent = useMemo((): SubagentScope | undefined => {
     if (!scopedAgent) return undefined;
     const parent = scopedAgent.parent;
     return {
       agent: scopedAgent,
-      rows: scopedRows ?? [],
+      rows: cardScopedRows ?? [],
       parent: parent === null ? chatName : (chatAgents.find((agent) => agent.id === parent)?.name ?? chatName),
       agents: descendants(chatAgents, scopedAgent.id),
       // As its "Subagent of" pill: back to its parent subagent's view, else the chat's.
       onBack: () => setSubagentView(parent ?? undefined),
     };
     // eslint-disable-next-line react-hooks/exhaustive-deps -- a new scope only when what it shows changes
-  }, [scopedKey, scopedRows, chatName, chatAgents]);
+  }, [scopedKey, cardScopedRows, chatName, chatAgents]);
   // How many rows Session Info's Background tasks lists for the view on screen, read as it reads
   // them (sessionInfoLists drops a transcript shell the registry no longer lists): a subagent's view
   // from its own rows (none yet: empty) and processes, with how its shells ended in the chat's rows

webviews/test/summary-held-while-scrolling.test.tsx

@@ -149,3 +149,27 @@ test("the card renders no more while history pages in under a 100-event scroll,
   act(() => root!.render(<Pane rows={["b1"]} chat="b" />));
   expect(host.textContent).toBe("b1");
 });
+
+test("subagent-scoped Session Info holds its rows; another subagent takes its own at once", async () => {
+  // App.tsx holds `snapshot.subagentViews[shown]` keyed by chat and subagent (cmux#29639), as it
+  // holds the chat's rows for the session card.
+  const { createRoot } = await import("react-dom/client");
+  const clock = manualClock();
+  const flag = new ReaderScroll(clock, HOLD_SETTLE_MS);
+  function Scoped({ rows, chat, subagent }: { rows: string[] | undefined; chat: string; subagent: string }) {
+    const held = useHeldWhileScrolling(rows, `${chat}\u0000${subagent}`, flag);
+    return <span>{(held ?? []).join(",")}</span>;
+  }
+  root = createRoot(host);
+  act(() => root!.render(<Scoped rows={["s5"]} chat="a" subagent="one" />));
+  act(() => flag.scrolled());
+  // Older history of the subagent pages in under the reader's scroll: Session Info keeps its rows.
+  act(() => root!.render(<Scoped rows={["s4", "s5"]} chat="a" subagent="one" />));
+  expect(host.textContent).toBe("s5");
+  act(() => clock.run());
+  expect(host.textContent).toBe("s4,s5");
+  // Another subagent of the same chat, mid-scroll: its own rows at once.
+  act(() => flag.scrolled());
+  act(() => root!.render(<Scoped rows={["t1"]} chat="a" subagent="two" />));
+  expect(host.textContent).toBe("t1");
+});