I was selecting a chunk of code, shift down-arrow until at the point I wanted. It suddenly jumped the selection down to or near the bottom of the tab, as if I had typed shift-control down, but I hadn’t. It was repeatable. If I typed shift up-arrow it went back where it was, and shift down-arrow popped back downward.
If I can find a repeatable example again, I’ll post it.
Weird.