Hi,

I am experiencing some weird behavior. I am simply trying to set the
vscroll. For that I first created a single overlay displaying a large
space and then use set-window-vscroll to set the vscroll.

It works when I simply load my Emacs configuration, but for some
reason it does not work when I start from emacs -Q.

Although I guess vscroll should be working on lines smaller than the
screen, just for testing I now display 3 'spaces' and then try to set
vscroll as follows:

(progn
  (pop-to-buffer "example")
  (dotimes (i 3)
    (let ((o (make-overlay (point)
                           (progn (insert " ")
                                  (point)))))
      (insert "\n")
      (overlay-put o 'display `(space . (:width (600) :height (800))))
      (overlay-put o 'face (list :background (pcase (% i 3)
                                               (0 "red")
                                               (1 "green")
                                               (2 "blue"))))))
  (goto-char (point-min))
   
  (set-window-vscroll nil 300 t))

I have tested it on Emacs 29.1 and 28.3.

I wonder if this is a bug, or if I am missing something.  Could you
please evaluate the above code starting from emacs -Q, and confirm
this is a bug (or tell me what I am missing)?

Thanks