bug#53275: 29.0.50; Cursor shows up at the middle of the screen after window start is forced when vscroll is set

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: 53275@debbugs.gnu.org
> Date: Sat, 15 Jan 2022 15:52:50 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >>   (progn
> >>     (set-window-vscroll nil 30 t)
> >>     (redisplay)
> >>     (set-window-start nil (point-max)))
> 
> > Sorry, I missed the "M-<" part.  With it, the behavior is
> > reproducible, but I'm not sure I understand what is "the correct
> > position" of the cursor in this case
> 
> It should show up where point is, which in this case is at the window
> start.
> 
> > or why does it matter what Emacs does in that case (except that it
> > should not crash).
> 
> Precision scrolling relies on this: when the buffer cannot be scrolled
> downwards sufficiently, it sets the window start to point-max and
> redisplays, which right now results in the cursor being displayed in the
> middle of the screen until the next redisplay.

Should be fixed now on the master branch.