I tried MS Edge. Works there.
Googled. Found that I'm not the first person to hit the problem, and a mention of ctrl-0 (cancel zoom) fixing the problem.
So I tried ctrl-0 on an image, and it reset it to fit the screen. Tried ctrl-0 on the observation window, and then opened another image, and it came out right. (I should have opened another image first to see if the first ctrl-0 has solved the problem.) So it now seems to be fixed.
Hypothesis - chrome remembers zoom settings for domains, and iSpot got set at 100% rather then no zoom (and these are treated differently).