If the row is expanded, gets the model holding the children of @self.
This model is the model created by the
[callback@Gtk.TreeListModelCreateModelFunc]
and contains the original items, no matter what value
[property@Gtk.TreeListModel:passthrough] is set to.
If the row is expanded, gets the model holding the children of @self.
This model is the model created by the [callback@Gtk.TreeListModelCreateModelFunc] and contains the original items, no matter what value [property@Gtk.TreeListModel:passthrough] is set to.