Re: Redisplay resets vscroll when window start changes

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: luangruo@yahoo.com,  mwd@md5i.com,  emacs-devel@gnu.org
> Date: Thu, 18 Nov 2021 22:17:41 -0500
> 
> > If you want to limit it only to those cases, then perhaps the answer
> > is yes, but the problem with those functions is that they try to
> > handle many use cases that are not completely described anywhere, so
> > testing whether a particular change could break any of them is nigh
> > impossible in practice.  And the code does work for the use cases for
> > which it was written.  Which is why I suggested a completely new
> > command for the purpose under discussion.
> 
> I must be missing something: in the test case posted here, AFAICT we
> just have a call to `previous-line` which doesn't involve any tall
> screen line, yet the result is still that vscroll is reset.

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.