In the view menu, there's an option called "Toggle Map Only." I selected that option to see what it looks like. This is what the entire QGIS window looks like in Map Only view:
As you can see, this option turns off all of the menus, panels and other visible controls. The only way to disable it is to use the hotkey combo. Of course, if you didn't memorize the hotkey combo, you can't find it again, because you don't have access to the View menu anymore.
How to un-toggle "Map Only" view mode?