The menu button is hampered by the fact that it's pretty tiny, also the menu items are tiny too. This may be OK for a menubar that's always visible (because it saves space and allows the user to aim beforehand), but it is bad for a hidden menu). There are no space constraints for the menu button or its popup. I think it should be much bigger by default, although an option would be okay too.
The available vertical space for the button is dictated by the height of the titlebar, We could make it wider though. But wouldn't that be a bit odd?