I took a stab at it, PTAL. Another option is to not do this, but document in both the howtos, and the documentationof Gtk::Widget that container-like custom widgets are not supported.
Thx,Baldvin
I have updated the custom container example in the gtkmm tutorial, https://gitlab.gnome.org/GNOME/gtkmm-documentation/-/tree/master/examples/book/custom/custom_container. Now it's more like a simplified vertical Box. It works well, except if it's a managed widget. I've noted this restriction in a comment in the source code. If you think it's too complicated the explain how to use a signal_destroy(), we can leave it at that for the moment.
Kjell