Tap gesture no longer add pointer after resetting (#52450)

* TapGestureRecognizer no longer adds the pointer after it resets.

* Adds an assertion to make sure TapGestureRecognizer.startTrackingPointer is never called when _down is null

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

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:vector_math/vector_math_64.dart' show Matrix4;
 import 'package:flutter/foundation.dart';
 
 import 'arena.dart';
@@ -183,13 +184,30 @@ abstract class BaseTapGestureRecognizer extends PrimaryPointerGestureRecognizer
 
   @override
   void addAllowedPointer(PointerDownEvent event) {
+    assert(event != null);
     if (state == GestureRecognizerState.ready) {
       // `_down` must be assigned in this method instead of `handlePrimaryPointer`,
       // because `acceptGesture` might be called before `handlePrimaryPointer`,
       // which relies on `_down` to call `handleTapDown`.
       _down = event;
     }
-    super.addAllowedPointer(event);
+    if (_down != null) {
+      // This happens when this tap gesture has been rejected while the pointer
+      // is down (i.e. due to movement), when another allowed pointer is added,
+      // in which case all pointers are simply ignored. The `_down` being null
+      // means that _reset() has been called, since it is always set at the
+      // first allowed down event and will not be cleared except for reset(),
+      super.addAllowedPointer(event);
+    }
+  }
+
+  @override
+  @protected
+  void startTrackingPointer(int pointer, [Matrix4 transform]) {
+    // The recognizer should never track any pointers when `_down` is null,
+    // because calling `_checkDown` in this state will throw exception.
+    assert(_down != null);
+    super.startTrackingPointer(pointer, transform);
   }
 
   @override

packages/flutter/test/gestures/tap_test.dart

@@ -4,8 +4,8 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
+import 'package:flutter_test/flutter_test.dart';
 
-import '../flutter_test_alternative.dart';
 import 'gesture_tester.dart';
 
 class TestGestureArenaMember extends GestureArenaMember {
@@ -904,4 +904,38 @@ void main() {
       expect(recognized, <String>['secondaryCancel']);
     });
   });
+
+  testGesture('A second tap after rejection is ignored', (GestureTester tester) {
+    bool didTap = false;
+
+    final TapGestureRecognizer tap = TapGestureRecognizer()
+      ..onTap = () {
+        didTap = true;
+      };
+    // Add drag recognizer for competition
+    final HorizontalDragGestureRecognizer drag = HorizontalDragGestureRecognizer()
+      ..onStart = (_) {};
+
+    final TestPointer pointer1 = TestPointer(1);
+
+    final PointerDownEvent down = pointer1.down(const Offset(0.0, 0.0));
+    drag.addPointer(down);
+    tap.addPointer(down);
+
+    tester.closeArena(1);
+
+    // One-finger moves, canceling the tap
+    tester.route(down);
+    tester.route(pointer1.move(const Offset(50.0, 0)));
+
+    // Add another finger
+    final TestPointer pointer2 = TestPointer(2);
+    final PointerDownEvent down2 = pointer2.down(const Offset(10.0, 20.0));
+    drag.addPointer(down2);
+    tap.addPointer(down2);
+    tester.closeArena(2);
+    tester.route(down2);
+
+    expect(didTap, isFalse);
+  });
 }