Make custom menu keybindings act like the defaults

[Harry Putnam]

Summary:

How can I get the effect of the default mouse menu bindings in a
keycombo that pops up menus?

Details:

Left mouse on root window gives main menu, another left mouse removes
it.

I used the suggested technique to get main menu to popup when the
windows key is pressed, and builtin window menu when `menu' key is
pressed:

Setting .Xmodmap:
  keycode 115 = F13
  keycode 116 = F14
  keycode 117 = F15

And in .xfwmrc:
 Key F13 A N Popup __builtin_root_menu__
 Key F14 A N Popup __builtin_root_menu__
 Key F15 A N Popup __builtin_window_menu__


But I still need the mouse to remove them.  How can I make it so
another press of window key or menu key will remove the popped up menu?