Emacs/Carbon: menubar is sometimes broken

[Ulrich Hobelmann]

Emacs 22.0.50.1 / Darwin 7.8.0
(newest CVS)

When I open another frame (C-x 5 2), and switch between the two frames 
for a while (clicking one frame titlebar, then the other, repeat), 
sometimes the menubar on top doesn't work anymore:
the menu title (file, options, etc.) lights up blue, but no menu 
appears.  The "Emacs" menu (i.e. leftmost) keeps working, though 
(probably since it's part of the OS?).