The only way to guarantee detection of a mouse leaving a control is with a 'mouseout' event -- something that no XL control supports. There might be some API calls that might help; but I don't know of any.
None of the ways that come to mind can guarantee detection of a mouse leaving a control. You could try:
(1) if the x or y coordinate is at (or close to) an edge, reset the control's format
(2) embed the real control inside a fake control. The fake control's mousemove event resets the format of the real control.
However, rapid mouse movement will not be detected. In my tests, I could fake XL out with either of the above techniques.