Thanks for this. This page has several things to look for if the problem raises its head again. I started over early this morning from a cold install. The problem is gone for the moment. By default, animations are turned on so I haven't dared turn them off. I shall be aware of things like zoom or an app that does this. What surprises me is that I kept creeping backwards through several snapshots I normally take with Timeshift. I couldn't shake it even with the snapshot I made right after the original install. Definitely some weird stuff going on.
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