Mouse messages while down

[Bill Marriott]

There's another fly in the ointment. Using "within" works reliably only if 
no buttons are overlapping (it doesn't respect layering like messages 
would), and the coordinate approach only works if the buttons are in a grid. 
If you have a more involved setup like using irregular graphics for the 
regions, invisible or disabled objects, etc it can get complicated.

The DragDrop family of messages would seem ideal; but those only work with 
text. :/