Brilliant! Thanks very much for posting that workaround. I had assumed that it was just a glitch in the Mac port of Stellarium.
On my computer, F11 also doesn't work for minimising or going to full screen. No big deal of course, as clicking on the icon in the menu at the bottom of the screen works fine. But it would be interesting to know if there is a combination of keys that does the job. Normally Fn+F11 would work, but not this time.