Comment 4 for bug 657782

Ignore my patch. Even on linux won't work well, as it disables all actions of the menu when it's hidden.
This is because shortcuts are assigned to actions binded to the menu and not to the main window, and Qt disables them if the widget is not active.

Jus, from what you describe, OSX doesn't need such thing (Windows too, maybe).
On Linux, this is hiding the menu independently of whether you are in fullscreen or not, so it's convenient to have this option for smaller screens.
But it's way more hassle than I thought, considering the ifdefs and the issue I described above.