Set-window-vscroll sometimes doesn't work

[Yuan Fu]

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?

Code:

(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)
    (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))))

Yuan

Image used in the code: