Thanks, everybody. Of course, I asked the question and then a ton of other work arrived, so it will be the first of the week before I can dive into trying the various things suggested.
I should note that when I log in to Plasma (from the Gnome-based Ubuntu login manager, which lists Gnome, Ubuntu desktop, TDE, and Plasma as possible sessions), in addition to a mouse cursor I get the window telling me that ProtonMail Bridge has started, as well as a little notification from Dropbox that I need to move my Dropbox directory to an ext4 partition, so a lot of stuff is starting. Also, all the non-Plasma desktops work as well as they normally would, and all report kwin as the window manager, so that upgrade clearly took place -- previously, it was Trinity's TDM. dep Sent withProtonMailSecure Email. Because privacy matters.
