void gtk_window_set_decorated ( GtkWindow* window, gboolean setting )
By default, windows are decorated with a title bar, resize
controls, etc. Some [window managers][gtk-X11-arch]
allow GTK+ to disable these decorations, creating a
borderless window. If you set the decorated property to
using this function, GTK+ will do its best to convince the window
manager not to decorate the window. Depending on the system, this
function may not have any effect when called on a window that is
already visible, so you should call it before calling gtk_widget_show().
On Windows, this function always works, since there’s no window manager policy involved.