gdk:window-fullscreen-mode (window)

window -- a toplevel gdk:window object
mode -- a value of the gdk:fullscreen-mode enumeration

Gets or sets the fullscreen mode applied to the window when fullscreen. Specifies whether the window should span over all monitors (in a multi-head setup) or only the current monitor when in fullscreen mode.

The mode argument is from the gdk:fullscreen-mode enumeration. If :on-all-monitors is specified, the fullscreen window will span over all monitors from the gdk:screen object.

On X11, searches through the list of monitors from the gdk:screen object the ones which delimit the 4 edges of the entire gdk:screen object and will ask the window manager to span the window over these monitors.

If the XINERAMA extension is not available or not usable, this function has no effect.

Not all window managers support this, so you can not rely on the fullscreen window to span over the multiple monitors when :on-all-monitors is specified.