Re: Redisplay resets vscroll when window start changes

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: luangruo@yahoo.com,  mwd@md5i.com,  emacs-devel@gnu.org
> Date: Fri, 19 Nov 2021 13:07:32 -0500
> 
> Eli Zaretskii [2021-11-19 09:13:53] wrote:
> > The current code is not prepared to deal with test cases like that
> > one.  The current Emacs code never sets vscroll unless we are moving
> > through a tall line, so resetting vscroll as part of previous-line is
> > a no-op when there's no tall lines in sight, and is not doing any
> > harm.  The test case was produced by a feature not yet in Emacs, and
> > so I'm saying that such a feature needs its own variant of
> > previous-line.
> 
> But to the extent than this feature does not intend to change the
> behavior of `previous-line`, this new variant of `previous-line` should
> ideally do everything exactly like the current `previous-line`, except
> be more careful not to reset vscroll, right?

No, not at all.  The current code consults the height of the line
through which it scrolls many times and for various purposes.  All of
those places can potentially do the wrong thing in the use case
described here.