A pointer that might cause a tap with a secondary button has contacted the screen at a particular location.
This is called after a short timeout, even if the winning gesture has not yet been selected. If the tap gesture wins, onSecondaryTapUp will be called, otherwise onSecondaryTapCancel will be called.
- kSecondaryButton, the button this callback responds to.
final GestureTapDownCallback? onSecondaryTapDown;