gdk:toplevel-modal (object)

object -- a gdk:toplevel object
modal -- true if the surface is modal, false otherwise

The accessor for the modal slot gets or sets whether the toplevel is modal. The application can use this hint to tell the window manager that a certain surface has modal behaviour. The window manager can use this information to handle modal surfaces in a special way.

You should only use this on surfaces for which you have previously called the gdk:toplevel-transient-for function.