ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Chapter 18. ActionScript Language Reference
Button._ymouse Property
Flash 6
vertical location of mouse pointer, relative to the button's registration point
read-only
theButton._ymouse
Description
The floating-point _ymouse property indicates the vertical location of the mouse pointer's hotspot, relative to theButton's registration point. To obtain a consistent _ymouse coordinate that is always measured relative to the Stage, use _root._ymouse.