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