[macOS] Handle interleaved movement and gesture events (flutter/engine#52201)

Fixes https://github.com/flutter/flutter/issues/146893

The crash happens, because with magic mouse, it is quite common to
receive `mouseExited:` event while the gesture event is still in
progress. This would reset the gesture state while the gesture event is
still active, leading to assertion when the gesture event ends.

The state machine assumes that when mouse exists that also ends any
gesture events. However that is not the case with magic mouse, which
handles the movement and gesture events separately.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterViewController.mm

@@ -129,7 +129,6 @@ struct MouseState {
     flutter_state_is_down = false;
     has_pending_exit = false;
     buttons = 0;
-    GestureReset();
   }
 };
 

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterViewControllerTest.mm

@@ -101,6 +101,7 @@
 - (bool)testFlagsChangedEventsArePropagatedIfNotHandled:(id)mockEngine;
 - (bool)testKeyboardIsRestartedOnEngineRestart:(id)mockEngine;
 - (bool)testTrackpadGesturesAreSentToFramework:(id)mockEngine;
+- (bool)mouseAndGestureEventsAreHandledSeparately:(id)engineMock;
 - (bool)testMouseDownUpEventsSentToNextResponder:(id)mockEngine;
 - (bool)testModifierKeysAreSynthesizedOnMouseMove:(id)mockEngine;
 - (bool)testViewWillAppearCalledMultipleTimes:(id)mockEngine;
@@ -287,6 +288,12 @@ TEST_F(FlutterViewControllerMockEngineTest, TestTrackpadGesturesAreSentToFramewo
       [[FlutterViewControllerTestObjC alloc] testTrackpadGesturesAreSentToFramework:mockEngine]);
 }
 
+TEST_F(FlutterViewControllerMockEngineTest, TestmouseAndGestureEventsAreHandledSeparately) {
+  id mockEngine = GetMockEngine();
+  ASSERT_TRUE(
+      [[FlutterViewControllerTestObjC alloc] mouseAndGestureEventsAreHandledSeparately:mockEngine]);
+}
+
 TEST_F(FlutterViewControllerMockEngineTest, TestMouseDownUpEventsSentToNextResponder) {
   id mockEngine = GetMockEngine();
   ASSERT_TRUE(
@@ -1004,6 +1011,42 @@ TEST_F(FlutterViewControllerTest, testViewControllerIsReleased) {
   return true;
 }
 
+// Magic mouse can interleave mouse events with scroll events. This must not crash.
+- (bool)mouseAndGestureEventsAreHandledSeparately:(id)engineMock {
+  FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
+                                                                                nibName:@""
+                                                                                 bundle:nil];
+  [viewController loadView];
+
+  // Test for pan events.
+  // Start gesture.
+  CGEventRef cgEventStart = CGEventCreateScrollWheelEvent(NULL, kCGScrollEventUnitPixel, 1, 0);
+  CGEventSetType(cgEventStart, kCGEventScrollWheel);
+  CGEventSetIntegerValueField(cgEventStart, kCGScrollWheelEventScrollPhase, kCGScrollPhaseBegan);
+  CGEventSetIntegerValueField(cgEventStart, kCGScrollWheelEventIsContinuous, 1);
+  [viewController scrollWheel:[NSEvent eventWithCGEvent:cgEventStart]];
+  CFRelease(cgEventStart);
+
+  CGEventRef cgEventUpdate = CGEventCreateCopy(cgEventStart);
+  CGEventSetIntegerValueField(cgEventUpdate, kCGScrollWheelEventScrollPhase, kCGScrollPhaseChanged);
+  CGEventSetIntegerValueField(cgEventUpdate, kCGScrollWheelEventDeltaAxis2, 1);  // pan_x
+  CGEventSetIntegerValueField(cgEventUpdate, kCGScrollWheelEventDeltaAxis1, 2);  // pan_y
+  [viewController scrollWheel:[NSEvent eventWithCGEvent:cgEventUpdate]];
+  CFRelease(cgEventUpdate);
+
+  NSEvent* mouseEvent = flutter::testing::CreateMouseEvent(0x00);
+  [viewController mouseEntered:mouseEvent];
+  [viewController mouseExited:mouseEvent];
+
+  // End gesture.
+  CGEventRef cgEventEnd = CGEventCreateCopy(cgEventStart);
+  CGEventSetIntegerValueField(cgEventEnd, kCGScrollWheelEventScrollPhase, kCGScrollPhaseEnded);
+  [viewController scrollWheel:[NSEvent eventWithCGEvent:cgEventEnd]];
+  CFRelease(cgEventEnd);
+
+  return true;
+}
+
 - (bool)testViewWillAppearCalledMultipleTimes:(id)engineMock {
   FlutterViewController* viewController = [[FlutterViewController alloc] initWithEngine:engineMock
                                                                                 nibName:@""