hydrurga: I have not rolled back with TimeShift, because I have no archive that I know preceded the problem. Moreover, if one of my software updates (via UpdateManager) caused the problem, I wouldn't know which one it was. Again, thank you for your attention and your suggestion.

boombaby: Very sharp, to notice that 18.2! I've bounced between 18.2 and 19 so much that this was bound to happen. However, there's only one physical machine, so the only difference between my 18.2 inxi data and my 19 data is the Graphics software section, which should be:
all: I think I'll play around with the 'nomodeset' kernel param as time (and curiosity) permits, just to eliminate nouveau misbehavior.

Thanks again, everyone.