I believe the issue happens when the line you are moving the cursor from a line that is hscrolled to a line that is shorter than (frame_width - hscroll_margin), the issue occurs.  However, if the line you are moving the cursor to is greater than or equal to (frame_width - hscroll_margin), the problem does not occur.  This is more evident when you set the hscroll-margin to something smaller, like 1 or 2.

The patch I attached has modified the if statement that has this condition in it (point >= frame_width - hscroll_margin) or something like that, I don't have the code in front of me.

On Tue, Aug 23, 2022 at 11:07 AM Eli Zaretskii <eliz@gnu.org> wrote:

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: Erik van Zwol <thejofat@gmail.com>,  57343@debbugs.gnu.org
> Date: Tue, 23 Aug 2022 18:56:34 +0200
>
> > I feel there's something else at work here, because if you move the
> > cursor up instead of down (i.e., to the previous instead of the next
> > line), the problem doesn't happen.
>
> I see the same hscrolling when moving to the next and to the previous
> line, both the same length (too short for hscrolling), see the attached
> screenshots.

You are saying that what you see on your system invalidates my
observations on my system?

All I'm saying is that what I see here tells me there's more here than
meets the eye.  AFAIU, your evidence actually affirms my feeling.