Allow trackpad inertia cancel events (#108190)

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

@@ -1718,7 +1718,7 @@ abstract class PointerSignalEvent extends PointerEvent {
     super.device,
     super.position,
     super.embedderId,
-  }) : assert(!identical(kind, PointerDeviceKind.trackpad));
+  });
 }
 
 mixin _CopyPointerScrollEvent on PointerEvent {
@@ -1787,7 +1787,8 @@ class PointerScrollEvent extends PointerSignalEvent with _PointerEventDescriptio
        assert(kind != null),
        assert(device != null),
        assert(position != null),
-       assert(scrollDelta != null);
+       assert(scrollDelta != null),
+       assert(!identical(kind, PointerDeviceKind.trackpad));
 
   @override
   final Offset scrollDelta;

packages/flutter/test/gestures/events_test.dart

@@ -848,12 +848,14 @@ void main() {
   test('Ensure certain event types are allowed', () {
     // Regression test for https://github.com/flutter/flutter/issues/107962
     expect(const PointerHoverEvent(kind: PointerDeviceKind.trackpad), isNotNull);
-
+    // Regression test for https://github.com/flutter/flutter/issues/108176
+    expect(const PointerScrollInertiaCancelEvent(kind: PointerDeviceKind.trackpad), isNotNull);
     // The test passes if it compiles.
   });
 
   test('Ensure certain event types are not allowed', () {
     expect(() => PointerDownEvent(kind: PointerDeviceKind.trackpad), throwsAssertionError);
+    expect(() => PointerScrollEvent(kind: PointerDeviceKind.trackpad), throwsAssertionError);
   });
 }