gdk:window-modal-hint (window)

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

Whether or not the window has the modal hint set. The gdk:window-modal-hint function determines whether or not the window manager is hinted that window has modal behaviour.

The application can use this hint to tell the window manager that a certain window has modal behaviour. The window manager can use this information to handle modal windows in a special way.

You should only use this on windows for which you have previously called the gdk:window-set-transient-for function.