[Mac OS X] "Full Screen" menu item does not update (check/uncheck) when entering/exiting full screen
1) Enter full screen using regular window title bar controls (at least on OS X Yosemite and up, there are three buttons in the corner of the window: Close, Minimize, and Full Screen/Maximize).
2) Open the View menu, and there is no check next to "Full Screen."
3) Click the "Full Screen" menu option. A check appears next to it, and MuseScore stays in full screen.
4) Exit full screen using title bar controls.
5) Open the View menu, and there is still a check next to "Full Screen."
Secondary El Capitan issue
The above is the generic issue, which I'm guessing is platform-independent. Things get really weird with the new OS X El Capitan, where the operating system adds a full screen control to the View menu if an application does not already have it. MuseScore's "Full Screen" command is not recognized, leading to this (look at the bottom of the menu):