....but but still solves the OP's problem.
Do you mean "F" would still solve the problem? No, it would not. If he is in the configuration where the OS bar (close, etc.) is not displayed, "F" will go to complete fullscreen (nothing but image) and back to same configuration. It will not return the OS bar.
Tip to the OP: I normally leave LR in the third configuration - no OS bar and the menu bar hidden until you mouse up to the top of the screen. When I want to close the application I mouse up to reveal the menu bar and select Exit from the File menu or I simply use Ctrl+Q. Only on the rare occasion that I want to down-size the application window (to drag it from one monitor to another, for instance) do I use Shift+F to return the OS bar.