Справка LibreOffice 7.5
Shows or hides the menus and toolbars in Writer or Calc. To exit the full screen mode, click the Full Screen button or press the Esc key.
In Writer and Calc, you can also use the shortcut keys Shift+J to switch between the normal and full screen mode.+
You can still use shortcut keys in Full Screen mode, even though the menus are unavailable.