Retrieves the current fullscreen state of @window.
Note that since fullscreening is ultimately handled by the window
manager and happens asynchronously to an application request, you
shouldn’t assume the return value of this function changing
immediately (or at all), as an effect of calling
[method@Gtk.Window.fullscreen] or [method@Gtk.Window.unfullscreen].
If the window isn't yet mapped, the value returned will whether the
initial requested state is fullscreen.
Retrieves the current fullscreen state of @window.
Note that since fullscreening is ultimately handled by the window manager and happens asynchronously to an application request, you shouldn’t assume the return value of this function changing immediately (or at all), as an effect of calling [method@Gtk.Window.fullscreen] or [method@Gtk.Window.unfullscreen].
If the window isn't yet mapped, the value returned will whether the initial requested state is fullscreen.