help tracking down gui bug please

[Richard Frith-Macdonald]

There is a fairly well known bug with pulldown menus, that sometimes the gui 
fails to track mouse movements (or something) so that the highlighted menu 
item does not change as the mouse is moved over the menu, and releasing the 
mouse button when  over an item therefore does not cause that item to be 
selected.

I've had it happen to me in GNUmail ... but not under any reproducible 
circumstances, and never at a point where it was convenient to try and start 
debugging.

Can anyone tell me how to reliably reproduce this bug?  I'd like to make an 
attempt at tracking it down and fixing it, but I can't seem to reproduce it.