|Thank you all for having an answer. I'll try the driver removal with a restart method. That sounds pretty solution-ey to me.|
Incidentally, there's nothing unusual about either of the mice. They're just garden variety wired USB, one a DYNEX (the old one) and the other a Logitech. Both work pefectly well in any of my USB ports in Linux, and they also work fine on each of the several other computers around my house (Windows, Mac, Chromebook). So probably my messy Windows is the culprit.