A pointer has contacted the screen at a particular location with a secondary button, which might be the start of a secondary tap.
This triggers after the down event, once a short timeout (deadline) has elapsed, or once the gestures has won the arena, whichever comes first.
- kSecondaryButton, the button this callback responds to.
- onTapDown, a similar callback but for a primary button.
- onTertiaryTapDown, a similar callback but for a tertiary button.
- TapDownDetails, which is passed as an argument to this callback.
- GestureDetector.onSecondaryTapDown, which exposes this callback.