A Pen is a type of pointer input that has most of the same behavior as a mouse, but can
also have event properties unique to a stylus. Additionally, while a mouse
has 5 buttons, a pen has 3 equivalent button states:
0 — Touch Contact (the default; equivalent to a left click)
2 — Barrel Button (equivalent to a right click)
5 — Eraser Button (currently unsupported by drivers)
