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: Wed, 17 Nov 2021 15:40:39 -0500
> 
> >> Would there be a way to distinguish the case where line-move can return
> >> a correct result without changing vscroll, from the case where it does
> >> require changing vscroll in order to find its answer?
> > I don't think I understand the question.
> 
> The question can be rephrased as: could `line-move` only reset vscroll
> in the case where we're scrolling through a very tall image?

It currently handles the case of a tall screen line, which could be
due to an image, an xwidget, or simply very large font.

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.