Fetch latest timeline events before exporting Performance data

packages/devtools_app/lib/src/screens/performance/panes/controls/performance_controls.dart

@@ -148,7 +148,10 @@ class _SecondaryPerformanceControls extends StatelessWidget {
         const SizedBox(width: denseSpacing),
         OpenSaveButtonGroup(
           screenId: ScreenMetaData.performance.id,
-          onSave: (_) => controller.exportData(),
+          onSave: (_) async {
+            await controller.fetchFullDataBeforeExport();
+            controller.exportData();
+          },
         ),
         if (isFlutterApp) ...[
           const SizedBox(width: denseSpacing),

packages/devtools_app/lib/src/screens/performance/panes/timeline_events/timeline_events_controller.dart

@@ -342,6 +342,18 @@ class TimelineEventsController extends PerformanceFeatureController
     await loadPerfettoTrace();
   }
 
+  /// Pulls any timeline events from the VM that have not been received yet.
+  ///
+  /// Unlike [forceRefresh], this does not reload the Perfetto trace viewer.
+  Future<void> pullLatestTimelineEvents() async {
+    try {
+      await _pullPerfettoVmTimeline();
+    } catch (e, st) {
+      // Don't block the export of the data we already have.
+      _log.warning('Failed to pull the latest timeline events.', e, st);
+    }
+  }
+
   void processTrackEvents() {
     if (!_isFlutterAppHelper()) {
       debugTraceCallback(

packages/devtools_app/lib/src/screens/performance/performance_controller.dart

@@ -291,6 +291,20 @@ class PerformanceController extends DevToolsScreenController
     ).toJson(),
   );
 
+  /// Pulls the latest timeline events so that they are included in the export.
+  ///
+  /// Timeline events are only polled periodically, so the most recent events
+  /// would otherwise be missing from the export.
+  Future<void> fetchFullDataBeforeExport() async {
+    final serviceManager = serviceConnection.serviceManager;
+    if (offlineDataController.showingOfflineData.value ||
+        !serviceManager.connectedState.value.connected ||
+        (serviceManager.connectedApp?.isDartWebAppNow ?? false)) {
+      return;
+    }
+    await timelineEventsController.pullLatestTimelineEvents();
+  }
+
   @override
   FutureOr<void> releaseMemory({bool partial = false}) async {
     await clearData(partial: partial, clearVmTimeline: !partial);

packages/devtools_app/test/screens/performance/performance_controller_test.dart

@@ -49,5 +49,17 @@ void main() {
       expect(controller.flutterFramesController.isActiveFeature, isTrue);
       expect(controller.timelineEventsController.isActiveFeature, isTrue);
     });
+
+    test('fetchFullDataBeforeExport does not pull timeline events when showing '
+        'offline data', () async {
+      final mockServiceManager =
+          mockServiceConnection.serviceManager as MockServiceManager;
+      clearInteractions(mockServiceManager);
+
+      await controller.fetchFullDataBeforeExport();
+
+      verifyNever(mockServiceManager.service);
+      expect(controller.timelineEventsController.fullPerfettoTrace, isEmpty);
+    });
   });
 }

packages/devtools_app/test/screens/performance/timeline_events/timeline_events_controller_test.dart

@@ -82,6 +82,32 @@ void main() {
       );
     });
 
+    group('pullLatestTimelineEvents', () {
+      setUp(() {
+        setGlobal(PreferencesController, PreferencesController());
+      });
+
+      test('adds the latest VM timeline to the trace', () async {
+        expect(eventsController.fullPerfettoTrace, isEmpty);
+
+        await eventsController.pullLatestTimelineEvents();
+
+        expect(eventsController.fullPerfettoTrace, isNotEmpty);
+      });
+
+      test('completes without throwing when the pull fails', () async {
+        // This fake service throws when the VM timeline is requested because
+        // it was not provided any timeline data.
+        setGlobal(ServiceConnectionManager, FakeServiceConnectionManager());
+
+        await expectLater(
+          eventsController.pullLatestTimelineEvents(),
+          completes,
+        );
+        expect(eventsController.fullPerfettoTrace, isEmpty);
+      });
+    });
+
     group('can clearData', () {
       setUp(() async {
         // Set some data in the controller. It does not matter if it is connected