[AccessD] How to tell a button's "air space" is no longer being hovered over

I have some code which changes the caption of a button when the user has the
ctrl key pressed, set during the mousemove event for the button.

I want however, that when the user leaves the "air space" for the button,
then the caption reverts - regardless whether they still have shift held.

I know I can use use the mousemove event associated with all surrounding
controls to reset it, but I would like something more related to the control
itself. 

Is there a method of testing that the user has moved the mouse outside the
button's air space? 

I have a feeling I am going to be out of luck...