Quite a few applications use GTK_WINDOW_POPUP to create various
temporary windows and place then on screen. That works fine on X11 but
on Wayland there is no global coordinate system for regular surfaces.
If the application is using a gdk temp window and set a parent with
gtk_window_transient_for(), the gdk wayland backend has all it needs to
create a subsurface that can be placed at will by the application.
Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=759738
