When the mouse is clicked or dragged, a pointer to this structure is passed to your widget function.

typedef struct {
     int                       x;
     int                       y;
     // Mouse button number, left = 0 (right button not yet supported.
     int                       button;
     // Scroll wheel delta (button in this case would be the wheel axis number).
     int                       delta;
} XPMouseState_t;