gdk-window-set-transient-for (window parent)

window -- a toplevel gdk-window object
parent -- another toplevel gdk-window object

Indicates to the window manager that window is a transient dialog associated with the application window parent. This allows the window manager to do things like center window on parent and keep window above parent.

See the function gtk-window-transient-for if you are using gtk-window or gtk-dialog widgets.