The pointer has contacted the screen with a secondary button, which might be the start of a long-press.
This triggers after the pointer down event.
- kSecondaryButton, the button this callback responds to.
- onLongPressDown, a similar callback but for a secondary button.
- onTertiaryLongPressDown, a similar callback but for a tertiary button.
- LongPressGestureRecognizer.onSecondaryLongPressDown, which exposes this callback at the gesture layer.
final GestureLongPressDownCallback? onSecondaryLongPressDown;