bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> Cc: 56008@debbugs.gnu.org
> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Thu, 16 Jun 2022 14:04:54 +0200
> 
> >     1. emacs -Q
> >     2. Visit attached image file.
> >     3. s o ;; to make the window show only some part of the image
> >     4. M->
> >     5. C-x 3 ;; split the frame into two windows
> >     6. M-s M-w search for something here RET
> >     7. C-x o ;; switch to the image-mode buffer
> >     8. Observe image-mode buffer moving to the beginning of image when
> >        the eww buffer gets updated with the webpage content.
> 
> I can reproduce this -- what a strange error.

Indeed.

> Hm...  anybody got any ideas how to debug this?  Something is apparently
> resetting the window-start in the wrong window?  Or...  something else?

Something else, because window-start stays put at 1.  image-mode
implements "M->" by setting vscroll and hscroll, it doesn't change
window-start.  Moreover, if you type "M-x" after the recipe, the image
gets redisplayed as expected after "M->".  Which means that the
window's vscroll didn't change, either.