OK, I was able to capture it with
$ sleep 22; import -window root bad.jpg
then pressing three right or left arrows (cursor movement), or one up or
one down arrow clears it, and I then made good.jpg. A single CTRL+L
doesn't always clear it.

It only affects my 32 bit fifteen year old Thinkpad R50e, so emacs 26.1
is going too fast, not confirming each rendering step has completed or
something.