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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.
If this recognizer doesn't win the arena, onSecondaryTapCancel is called next. Otherwise, onSecondaryTapUp is called next.
- 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.