Urgh, too late I manage to crack Gnome's search engine. This seems to appear upstream in a few contexts, but I don't understand what the consequences are for a fix in Debian:
https://gitlab.gnome.org/GNOME/gnome-shell/issues/773 Closed; says it's applications' fault! https://gitlab.gnome.org/GNOME/mutter/issues/602 .. points to https://gitlab.gnome.org/GNOME/gtk/issues/539 .. where the same guy says it's applications' fault. https://gitlab.gnome.org/GNOME/gtk/issues/1895 .. where it looks as if the 'lower window' implementation has never been completed. C.
