Menu positioning query

I’m playing with FvwmButtons, and I’d like the first button to bring up a menu immediately above that button. I’ve been reading the manual page section on menu positioning several times and have so far not understood it properly.

My buttons are all 64x64 pixels, so the bottom-left corner of the menu should appear at +0-64. So I was hoping that the following would position the menu properly:

*FvwmButtons: (Action 'Menu ButtonMenu Rectangle +0-64 +0 -100m', Icon /usr/share/icons/wm-icons/48x48-gnome/linux.xpm, ActiveTitle 'Menu')

My understanding of the command “Menu … Rectangle +0-64 +0 -100m” would be to position the menu with its top left at +0-64, then move the menu up by 100% of the menu’s height; therefore the bottom left corner should now be at +0-64. This isn’t happening: the bottom left of the menu is at the bottom left of the screen, covering up the menu button.

If I instead use “Menu … Mouse +0 -100m” that works as expected; the bottom left of the menu is exactly on the mouse pointer. But I can’t get this offset to work with a defined rectangle. Can anyone tell me what I’m doing wrong?