cascaded menus no longer display until mouse is released (Tk 8.5.8 or 8.5.9 change?)

When I upgraded tcl/tk from 8.5.7 to 8.5.9, cascaded menus no longer display
until I release the mouse button (while the mouse is over the name of the menu).

So if an exmh menu contains several cascaded menus (for example, the folder's
'More' menu), you can no longer simply move the mouse through that menu (with
the left button depressed) to view the items in each of the cascaded menus.
Instead, you must stop at each item and release the left button to cause that
cascaded menu to appear. To proceed to the another cascaded menu, you must then
depress the mouse button again, drag down to the next cascaded menu, then
release the mouse button again.

This doesn't seem to be an exmh issue; it seems to be Tk.

Looking through Tk ChangeLog and bug reports, I can't determine if this is a
Tk bug or if it is a new feature introduced in Tk 8.5.8 or 8.5.9. If it's a
new Tk behavior, is there a change I can make in exmh to restore the old cascaded
menu behavior?

Platform:
exmh version 2.7.2 pulled Jan 7 2011 from http://exmh.cvs.sourceforge.net/viewvc/exmh/exmh/
Tcl/Tk 8.5.9
Solaris 9 & 10