A pointer that previously triggered onLongPressDown will not end up causing a long-press.
This triggers once the gesture loses if onLongPressDown has previously been triggered.
If the user completed the long-press, and the gesture won, then onLongPressStart and onLongPress are called instead.
- kPrimaryButton, the button this callback responds to.
- LongPressGestureRecognizer.onLongPressCancel, which exposes this callback at the gesture layer.
final GestureLongPressCancelCallback? onLongPressCancel;