Question about weird vscroll behavior

[dalanicolai <dalanicolai@gmail.com>]

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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

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