Eli Zaretskii <eliz@gnu.org> writes:

> Then some redisplay optimization is at work here.  If your build was
> configured with --enable-checking=glyphs, does the problem go away if
> you set _all_ of the following variable non-nil?
>
>   inhibit-try-cursor-movement
>   inhibit-try-window-id
>   inhibit-try-window-reusing

No, that doesn't change anything.

But I now see more clearly what's not being updated -- it copies all the
bits in the background on the line that's being moved.

Here's the display first: