A pointer has contacted the screen at a particular location with a primary button, which might be the start of a tap.
This triggers after the down event, once a short timeout (kPressTimeout) has elapsed, or once the gestures has won the arena, whichever comes first.
The position of the pointer is provided in the callback's
argument, which is a TapDragDownDetails object.
The number of consecutive taps, and the keys that were pressed on tap down
are also provided in the callback's