bug#57343: hscroll current-line scrolls short lines too far to the left

[Eli Zaretskii <eliz@gnu.org>]

> From: Erik van Zwol <thejofat@gmail.com>
> Date: Sun, 21 Aug 2022 22:03:29 -0600
> 
> starting from 'emacs -Q':
> Toggle-truncate-lines (so that long lines are truncated)
> Evaluate the following:
> (setq auto-hscroll-mode 'current-line)
> (setq hscroll-margin 5)
> (setq hscroll-step 1)
> Insert enough text such that the line starts to hscroll
> On the line directly below, insert enough text to fill about half the
> width of the frame
> Move point to the long line near the end (must be hscrolling the single
> line)
> Move point down to the next line, it should hscroll even though it does
> not need to
> See the animated gifs on this post:
> https://www.reddit.com/r/emacs/comments/wgef0b/looking_for_better_alternative_to_autohscrollmode/
> 
> I have attached a patch that I think fixes the issue.  Please tell me
> how to proceed, and what other information you need.

Thanks.

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.

So I need to look closer at what is going on and why.  Stay tuned.