By default, windows have a close button in the window frame. Some [window managers][gtk-X11-arch] allow GTK+ to disable this button. If you set the deletable property to FALSE using this function, GTK+ will do its best to convince the window manager not to show a close button. 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 Widget.show.
On Windows, this function always works, since there’s no window manager policy involved.