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Called when a pointer has contacted the screen at a particular location with a primary button, which might be the start of a long-press.
This triggers after the pointer down event.
- kPrimaryButton, the button this callback responds to.
- onSecondaryLongPressDown, a similar callback but for a secondary button.
- onTertiaryLongPressDown, a similar callback but for a tertiary button.
- LongPressDownDetails, which is passed as an argument to this callback.
- GestureDetector.onLongPressDown, which exposes this callback in a widget.