Fix for #112403 and b/249091367 (#121615)

Fix monodrag gestures for #112403 and b/249091367

Author: dnfield

packages/flutter/lib/src/gestures/monodrag.dart

@@ -222,6 +222,24 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
   late OffsetPair _initialPosition;
   late OffsetPair _pendingDragOffset;
   Duration? _lastPendingEventTimestamp;
+
+  /// When asserts are enabled, returns the last tracked pending event timestamp
+  /// for this recognizer.
+  ///
+  /// Otherwise, returns null.
+  ///
+  /// This getter is intended for use in framework unit tests. Applications must
+  /// not depend on its value.
+  @visibleForTesting
+  Duration? get debugLastPendingEventTimestamp {
+    Duration? lastPendingEventTimestamp;
+    assert(() {
+      lastPendingEventTimestamp = _lastPendingEventTimestamp;
+      return true;
+    }());
+    return lastPendingEventTimestamp;
+  }
+
   // The buttons sent by `PointerDownEvent`. If a `PointerMoveEvent` comes with a
   // different set of buttons, the gesture is canceled.
   int? _initialButtons;
@@ -363,7 +381,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     if (_state != _DragState.accepted) {
       _state = _DragState.accepted;
       final OffsetPair delta = _pendingDragOffset;
-      final Duration timestamp = _lastPendingEventTimestamp!;
+      final Duration? timestamp = _lastPendingEventTimestamp;
       final Matrix4? transform = _lastTransform;
       final Offset localUpdateDelta;
       switch (dragStartBehavior) {
@@ -449,7 +467,7 @@ abstract class DragGestureRecognizer extends OneSequenceGestureRecognizer {
     }
   }
 
-  void _checkStart(Duration timestamp, int pointer) {
+  void _checkStart(Duration? timestamp, int pointer) {
     if (onStart != null) {
       final DragStartDetails details = DragStartDetails(
         sourceTimeStamp: timestamp,

packages/flutter/test/gestures/monodrag_test.dart

@@ -10,6 +10,31 @@ import 'gesture_tester.dart';
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
+  test('acceptGesture tolerates a null lastPendingEventTimestamp', () {
+    // Regression test for https://github.com/flutter/flutter/issues/112403
+    // and b/249091367
+    final DragGestureRecognizer recognizer = VerticalDragGestureRecognizer();
+    const PointerDownEvent event = PointerDownEvent(timeStamp: Duration(days: 10));
+
+    expect(recognizer.debugLastPendingEventTimestamp, null);
+
+    recognizer.addAllowedPointer(event);
+    expect(recognizer.debugLastPendingEventTimestamp, event.timeStamp);
+
+    // Normal case: acceptGesture called and we have a last timestamp set.
+    recognizer.acceptGesture(event.pointer);
+    expect(recognizer.debugLastPendingEventTimestamp, null);
+
+    // Reject the gesture to reset state and allow accepting it again.
+    recognizer.rejectGesture(event.pointer);
+    expect(recognizer.debugLastPendingEventTimestamp, null);
+
+    // Not entirely clear how this can happen, but the bugs mentioned above show
+    // we can end up in this state empircally.
+    recognizer.acceptGesture(event.pointer);
+    expect(recognizer.debugLastPendingEventTimestamp, null);
+  });
+
   testGesture('do not crash on up event for a pending pointer after winning arena for another pointer', (GestureTester tester) {
     // Regression test for https://github.com/flutter/flutter/issues/75061.