Managed to catch a gnome-shell message "cannot reopen /dev/dri/card0: device busy" when plugging a USB-C dongle for an external monitor. So I logged out, and restarted my session *after* plugging in the dongle - surely it won't need to fuck with the device then? - and it started working.
(This is a dongle for an external monitor that I had never managed to make work. Apparently all I needed was to plug it in before starting my session.)
(Why was GDM happy with plugging in the middle...?)

Sylvia
in reply to 📱CatSalad🐈🥗〰️𝔞𝔫𝔡𝔯𝔬𝔦𝔡ᵈᵉᵛ𝅭␎ • • •