bug#70038: 29.3.50; Shift up/down in buffer with images on M-x other-window with some fonts

[Eli Zaretskii]

> From: Rahguzar <rahguzar@zohomail.eu>
> Cc: Eli Zaretskii <eliz@gnu.org>, 70038@debbugs.gnu.org
> Date: Thu, 28 Mar 2024 18:24:32 +0100
> 
> I can also reproduce this now! And vertico mode can be replaced with the
> builtin icomplete-vertical-mode. So, the following recipe starting with
> emacs -Q works for me:
> 
> 1) Paste
> (let ((height (/ (* 2 (frame-pixel-height)) 15)))
>   (icomplete-vertical-mode)
>   (defun pin-vscroll-down (win)
>     (set-window-vscroll win (/ height 2) t))
>   (let ((image1 (create-image "~/Downloads/image1.png" nil nil :height 
> height))
>         (image2 (create-image "~/Downloads/image2.png" nil nil :height 
> height))
>         (image3 (create-image "~/Downloads/image3.png" nil nil :height 
> height)))
>     (with-current-buffer (get-buffer-create "*image-scroll-test*")
>       (insert " \n \n \n \n \n \n")
>       (put-image image1 1)
>       (put-image image2 5)
>       (put-image image3 9)
>       ;; With larger image sizes (goto-char 3)
>       ;; also consistently triggers the problem.
>       (goto-char 11)
>       (add-hook 'pre-redisplay-functions #'pin-vscroll-down nil t))
>     (split-window-right)
>     (other-window 1)
>     (switch-to-buffer "*image-scroll-test*")))
> 
> into scratch buffer.
> 
> 2) Evaluate the form above using `C-M-x`.
> 
> 3) Type M-x t
> 
> 4) Wait till minibuffer expands to show completions, then type `C-g` to
> quit minibuffer.
> 
> 5) Typing `C-x 0` results in the window with images losing vscroll.

Po Lu, I'm looking at this part of redisplay_window:

   force_start:

    /* Handle case where place to start displaying has been specified,
       unless the specified location is outside the accessible range.  */
    if (w->force_start)
      {
        /* We set this later on if we have to adjust point.  */
        int new_vpos = -1;

        w->force_start = false;

        /* The vscroll should be preserved in this case, since
           `pixel-scroll-precision-mode' must continue working normally
           when a mini-window is resized.  (bug#55312) */
        if (!w->preserve_vscroll_p || !window_frozen_p (w))  <<<<<<<<<<<<<<<
          w->vscroll = 0;

        w->preserve_vscroll_p = false;
        w->window_end_valid = false;

where you added the condition for resetting w->vscroll in commit
fd8eaa72a61, and I'm thinking that perhaps the condition should be

        if (!w->preserve_vscroll_p && !window_frozen_p (w))

instead?  If not, can you explain why we use OR and not AND there?

Ramon, if you replace "||" with "&&" in the above condition, does the
problem go away for you also when you change fonts, as you described
in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=70038#29 ?