Moves the window out of fullscreen mode. If the window was not fullscreen, does nothing.
On X11, asks the window manager to move window out of the fullscreen state, if the window manager supports this operation. Not all window managers support this, and some deliberately ignore it or don’t have a concept of “fullscreen”; so you can’t rely on the unfullscreenification actually happening. But it will happen with most standard window managers, and GDK makes a best effort to get it to happen.