From FreeCAD Documentation
- To exit fullscreen mode: first click in the correct 3D View to activate it.
- There are several ways to invoke the command:
- Select the View → Document Window →
Fullscreen option from the menu. This option does not work if you want to switch off fullscreen mode and there are other docked 3D Views.
- Select the Document Window →
Fullscreen option from the 3D View context menu.
- Use the keyboard shortcut: F11.