Re: Set-window-vscroll sometimes doesn't work

[Yuan Fu]

> On Oct 18, 2020, at 10:34 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Sat, 17 Oct 2020 20:35:00 -0400
>> 
>> Normally if you scroll back onto an image, the image appears completely. I 
>> want to instead only show part of the image, like what pixel scrolling would 
>> do. If everything works as expected I should only see the bottom of the 
>> image, instead I see the whole image.
>> 
>> It seems to be because (set-window-vscroll nil (- img-height 
>> (frame-char-height)) t) didn’t take effect, if you call that again with M-: 
>> or wrap it in (run-with-timer), then it works as expected.
>> 
>> Is this caused by some redisplay optimization or something?
> 
> No.  The problem is in your code.  First, you use set-window-start and
> scroll-down, both of which tramp window-vscroll (as you probably
> realize if you think about what those do).  

Why is so? Is there any material that I can read about these?

> And after all that, you
> need one "normal" redisplay cycle before you can set the vscroll, so,
> for example, a single change below should make the code work as you
> expect.
> 
> (progn
>  (switch-to-buffer (get-buffer-create "test"))
>  (erase-buffer)
>  (insert "\n\n")
>  (insert-image (create-image "abby road.jpeg" nil nil :scale 0.1)
>                "x")
>  (insert "\n")
>  (let ((after-img (point)))
>    (set-window-start nil (point))
>    (scroll-down 1)
>    (sit-for 0)  ;; <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
>    (let* ((img (plist-get (text-properties-at (window-start)) 'display))
>           (img-height (cdr (image-size img t))))
>      (set-window-vscroll nil (- img-height (frame-char-height)) t))))

This works, but brings flickers: the image first shows completely then the 
vscroll takes effect, which is reasonable but not preferred. I found that using 
(set-window-start nil (point) t) with the third argument non-nil gives me the 
desired effect. My guess is that, as the doctoring said, setting that to 
non-nil prevents redisplay to try to make point completely visible, so 
redisplay doesn’t modify vscroll, so my vscroll value survived. Is that right?

Thanks,
Yuan