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