Event Mask

Has the following bit level definitions:

Bits 7-15

Reserved. Must be 0.
Bit 6
Set, if Button 3 is down.
Bit 5
Set, if motion, with Button 3 down.
Bit 4
Set, if Button 2 is down.
Bit 3
Set, if motion, with Button 2 down.
Bit 2
Set, if Button 1 is down.
Bit 1
Set, if motion, with Button 1 down.
Bit 0
Set, if all mouse motion, no buttons down.
A set bit has a value of 1.