bug#69972: 29.1; Unexpected behavior when scrolling images

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 24 Mar 2024 02:07:16 -0700
> From:  Joseph Turner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> When the image at point is larger than the current window and there is
> no content after the image, interactively scrolling down (with the
> scroll-up command) unexpectedly scrolls past the image to blankness.

I cannot reproduce this, I think.

> Test this by evaluating the following snippet then interactively running
> `scroll-up' repeatedly:
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil
>     ;; Scale the image more if it doesn't take up the whole window.
>     :scale 5))
>   (goto-char (point-max))
>   (pop-to-buffer (current-buffer)))

This recipe doesn't include the call to scroll-up, so I'm unsure how
you did that and what you saw.  When I try "M-: (scroll-up) RET" or
"M-x scroll-up RET", I get several scrolls by window-size, and then
"End of buffer" error when I hit the end of the buffer.  If this is
unexpected, please tell why.

> Even more unexpectedly, when point is before the image, running
> `scroll-up' repeatedly eventually scrolls back to the top of the image:
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil :scale 5))
>   (goto-char (point-min))
>   (pop-to-buffer (current-buffer)))

I do see this, but why is that a problem?  You supposed to use
scroll-up-command instead, which handles these marginal cases much
better.  scroll-up itself is not smart enough to avoid the perceived
"scroll back to top", which is actually caused by the fact that we
zero out window-vscroll (which is how we handle scrolling past large
images).