> emacs -Q
 >
 > ;; Evaluate this code:
 >
 > (defun bar ()
 >   (interactive)
 >   (message "%s :: %s" last-nonmenu-event
 >      (posn-window (event-start last-nonmenu-event)))
 >   (sit-for 5))
 >
 > (global-set-key
 >  [down-mouse-2]
 >  '(menu-item "foo"
 >              (keymap "foo"
 >                      (bar menu-item "bar" bar)
 >                      (submenu menu-item "submenu"
 >                               (keymap "submenu" (bar menu-item "bar" bar))))))
 >
 > C-x 2
 > Put "scratch" in one window, "messages" in the other.
 > Select "messages" window.
 > Move mouse over "scratch", but do not select it.
 > Press mouse-2, and select the "bar" menu-item.
 >
 >
 > When called from the top-level menu, `bar' correctly returns:
 >
 > (down-mouse-2 (#<window 3 on *scratch*> 534 (251 . 101) 2649593899 nil
 > 534 (27 . 6) nil (8 . 11) (9 . 15))) :: #<window 3 on *scratch*>
 >
 >
 > When called from the submenu, `bar' returns a bad result:
 >
 > bar :: #<window 7 on *Messages*>
 >
 > The window is wrong, etc.

Glenn, just for fun please try the attached patch.  It seems to solve
your problem here but I have no idea whether it has any detrimental
side-effects.

martin