perf(team): enable virtualization past threshold + tests

Final step of the virtualization plan. Turns the virtualized render
path on in production behind a row-count threshold, and adds regression
tests covering every gate.

- `VIRTUALIZATION_ROW_THRESHOLD = 60`. Short lists stay on the direct
  render path (no wrapper, no position: absolute, no measurement
  churn). Above the threshold the virtualizer takes over. Threshold is
  sized so conversations under ~one session of activity don't pay the
  virtualization cost; it activates once scrolling through a longer
  history.
- `shouldVirtualize` now requires `renderRows.length >= threshold` in
  addition to the existing opt-in and scroll-ref checks.
- `MessagesPanel` opts into virtualization for every layout it wires
  (inline / sidebar / bottom-sheet). The internal threshold then
  decides when to actually enable it, so callers don't need per-layout
  heuristics.
- Tests: adds a new `ActivityTimeline virtualization threshold` block
  covering (a) below-threshold list stays on the direct path,
  (b) no viewport → direct path regardless of count, (c) above
  threshold + viewport with `virtualizationEnabled` flips to the
  virtualized render path (simulated by clicking "show all" past
  pagination).

With this in, #70 → #74 combine to deliver:
- correct IntersectionObserver roots in scroll containers
- atomic render rows with stable keys
- windowed rendering with DOM-measured scrollMargin and measureElement
- auto-on when the cost of direct rendering actually shows up

src/renderer/components/team/activity/ActivityTimeline.tsx

@@ -136,6 +136,15 @@ const EMPTY_TEAM_COLOR_MAP = new Map<string, string>();
 const DEFAULT_COLLAPSE_MODE = 'default' as const;
 const VIRTUALIZER_OVERSCAN = 8;
 
+/**
+ * Row count above which virtualization is worth its complexity cost. Below
+ * this, the direct render path is both simpler and faster (no wrapper div,
+ * no position: absolute, no measurement churn). Chosen so conversations under
+ * roughly one session of activity stay on the direct path and the virtualized
+ * path only activates when scrolling behavior actually starts to matter.
+ */
+const VIRTUALIZATION_ROW_THRESHOLD = 60;
+
 /**
  * Per-kind height estimates for `estimateSize`. These are rough initial guesses
  * only; the virtualizer re-measures rows as they mount via `measureElement`
@@ -587,14 +596,15 @@ export const ActivityTimeline = React.memo(function ActivityTimeline({
     return rows;
   }, [pinnedThoughtGroup, previousSessionAnchorByIndex, startIndex, timelineItems]);
 
-  // Virtualizer gate — dormant unless the parent explicitly opts in via
-  // `viewport.virtualizationEnabled`. The contract carries this flag so the
-  // (large) virtualized render path can land before any caller flips the
-  // switch, and can be toggled on per-layout once measurement is validated.
+  // Virtualizer gate — activates only when the parent opts in via
+  // `viewport.virtualizationEnabled`, the scroll element ref is present, and
+  // the row count is large enough for virtualization to pay for itself. Below
+  // the threshold the direct render path is both simpler and faster, so we
+  // keep it for short lists.
   const shouldVirtualize =
     viewport?.virtualizationEnabled === true &&
     viewport.scrollElementRef != null &&
-    renderRows.length > 0;
+    renderRows.length >= VIRTUALIZATION_ROW_THRESHOLD;
 
   // DOM-measured distance from the scroll container's scroll origin to the
   // timeline root. Hand-summing composer/status/padding heights would drift as

src/renderer/components/team/messages/MessagesPanel.tsx

@@ -255,7 +255,10 @@ export const MessagesPanel = memo(function MessagesPanel({
       scrollElementRef: activeScrollContainerRef,
       observerRoot: activeScrollContainerRef,
       scrollMargin: 0,
-      virtualizationEnabled: false,
+      // Opt into virtualization; ActivityTimeline keeps the direct render
+      // path for short lists and only switches to the windowed path once
+      // the row count crosses its internal threshold.
+      virtualizationEnabled: true,
     };
   }, [activeScrollContainerRef]);
   const handleExpandContent = useCallback(() => {

test/renderer/components/team/activity/ActivityTimeline.test.ts

@@ -464,3 +464,129 @@ describe('ActivityTimeline viewport observerRoot', () => {
     scrollHost.remove();
   });
 });
+
+describe('ActivityTimeline virtualization threshold', () => {
+  let container: HTMLDivElement;
+
+  beforeEach(() => {
+    vi.stubGlobal('IS_REACT_ACT_ENVIRONMENT', true);
+    container = document.createElement('div');
+    document.body.appendChild(container);
+  });
+
+  afterEach(() => {
+    container.remove();
+    document.body.innerHTML = '';
+    vi.unstubAllGlobals();
+  });
+
+  const buildMany = (count: number): InboxMessage[] =>
+    Array.from({ length: count }, (_, i) =>
+      makeMessage({
+        messageId: `msg-${i}`,
+        text: `message ${i}`,
+        from: 'alice',
+        source: 'inbox',
+        leadSessionId: `member-session-${i}`,
+      })
+    );
+
+  it('does not enter the virtualized render path when the row count is below the threshold', async () => {
+    const scrollHost = document.createElement('div');
+    document.body.appendChild(scrollHost);
+    const scrollRef = { current: scrollHost };
+
+    const root = createRoot(container);
+    await act(async () => {
+      root.render(
+        React.createElement(ActivityTimeline, {
+          messages: buildMany(10),
+          teamName: 'demo-team',
+          viewport: {
+            scrollElementRef: scrollRef,
+            observerRoot: scrollRef,
+            scrollMargin: 0,
+            virtualizationEnabled: true,
+          },
+        })
+      );
+    });
+
+    // Virtualized path wraps items in an absolute-position container; the
+    // direct path does not. Assert the wrapper is absent.
+    const absoluteWrapper = container.querySelector<HTMLDivElement>('div[style*="position: relative"]');
+    expect(absoluteWrapper).toBeNull();
+    // Sanity check: direct render still emits at least one activity item.
+    expect(container.textContent).toContain('message 0');
+
+    await act(async () => {
+      root.unmount();
+    });
+    scrollHost.remove();
+  });
+
+  it('falls back to the direct render path when no viewport is provided', async () => {
+    const root = createRoot(container);
+    await act(async () => {
+      root.render(
+        React.createElement(ActivityTimeline, {
+          messages: buildMany(80),
+          teamName: 'demo-team',
+        })
+      );
+    });
+
+    const absoluteWrapper = container.querySelector<HTMLDivElement>('div[style*="position: relative"]');
+    expect(absoluteWrapper).toBeNull();
+    expect(container.textContent).toContain('message 0');
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
+  it('enters the virtualized render path when row count crosses the threshold', async () => {
+    const scrollHost = document.createElement('div');
+    document.body.appendChild(scrollHost);
+    const scrollRef = { current: scrollHost };
+
+    const root = createRoot(container);
+    await act(async () => {
+      root.render(
+        React.createElement(ActivityTimeline, {
+          messages: buildMany(80),
+          teamName: 'demo-team',
+          viewport: {
+            scrollElementRef: scrollRef,
+            observerRoot: scrollRef,
+            scrollMargin: 0,
+            virtualizationEnabled: true,
+          },
+        })
+      );
+    });
+
+    // Default pagination caps visible rows at 30, which stays below the
+    // threshold, so the direct render path is in effect here. Click "show
+    // all" to expose every message — that pushes row count past the gate.
+    const showAllButton = [...container.querySelectorAll('button')].find(
+      (b) => b.textContent?.toLowerCase().includes('show all')
+    );
+    expect(showAllButton).toBeDefined();
+
+    await act(async () => {
+      showAllButton?.click();
+    });
+
+    // Virtualized path: sized container div with `position: relative`
+    // directly inside the timeline root. jsdom serialises style attributes
+    // with spaces after the colon, so match case-insensitively.
+    const html = container.innerHTML;
+    expect(html.toLowerCase()).toMatch(/position:\s*relative/);
+
+    await act(async () => {
+      root.unmount();
+    });
+    scrollHost.remove();
+  });
+});