bug#69972: 29.1; Unexpected behavior when scrolling images

[Joseph Turner via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Joseph Turner <joseph@ushin.org> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> 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.
>
> On my machine, I don't get "End of buffer" error.

Correction - I *do* get "End of buffer" error, but only after fully
scrolling past the image (which IMO is unexpected).

> I tested three different ways in the following snippet
>
> (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))
>   (pixel-scroll-precision-mode -1)  ; Ensure mouse wheel scroll up and down works
>   (pop-to-buffer (current-buffer)))
>
> 1. "M-: (scroll-up) RET" (repeatedly)
> 2. C-v (repeatedly)
> 3. <wheel-down> (repeatedly)
>
> With all three methods, at first Emacs gradually scrolls the image, but
> then when I reach the bottom of the image, the image disappears entirely
> as Emacs scrolls past it all at once.
>
>>> 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).
>
> You're right.  This is not a problem in practice.  When point is before
> the image, both C-v and <wheel-down> produce the same behavior as above.
>
> Joseph