Tkinter: update_idletasks

[Jeffrey Barish]

I'm confused about how to use the update_idletasks method.  In my
program, I have a handler for a button in which execution will linger. 
During that time, I would like for the GUI to continue to show signs of
life.  I have a Pmw MessageBar in which I display a status message.  I
figured out that if I run update_idletasks on that MessageBar, then the
MessageBar will update the display as I update the message.  However,
if I cover the GUI with some other window and then expose it again, the
GUI does not refresh until the handler finishes (except for the
MessageBar).  Do I have to run the update_idletasks method for every
widget in the GUI?  for all the frames? for just the root frame?  Or is
it impossible to get the GUI to refresh in this situation?
-- 
Jeffrey Barish