The pointer has contacted the screen with a tertiary button, which might be the start of a long-press.
This triggers after the pointer down event.
If the user completes the long-press, and this gesture wins, onTertiaryLongPressStart will be called after this callback. Otherwise, onTertiaryLongPressCancel will be called after this callback.
- kTertiaryButton, the button this callback responds to.
- onLongPressDown, a similar callback but for a primary button.
- onSecondaryLongPressDown, a similar callback but for a secondary button.
- LongPressGestureRecognizer.onTertiaryLongPressDown, which exposes this callback at the gesture layer.
final GestureLongPressDownCallback? onTertiaryLongPressDown;