You can enter full-screen mode in Rubymine by navigating to MENU > VIEW > ENTER FULL SCREEN. Once in full-screen mode, moving your mouse to the top of the screen triggers the menu bar to dropdown, where you can once again navigate to MENU > VIEW > (now changed to) EXIT FULL SCREEN.
Does anyone know of a shortcut for this on Mac? I think there is one for OS X - not sure what it is off-hand, but can't find anyone anywhere for windows.
CTRL-COMMAND-F
OS X KeyMap cheatsheet here: http://www.jetbrains.com/ruby/docs/RubyMine_ReferenceCard_Mac.pdf