Show menubar in fullscreen
In SystemPreferences->General check "automatically hide and show the menubar".
Then it works like this: you do the app full screen with the green button, the menubar is hidden, move the mouse pointer to the top of the screen and the menubar becomes visible above everything.
added: It works also even when you do not check "automatically hide and show the menubar", a tid bit slower perhaps.
added; you can also set a key combination if you want.