Haha, okay Eli. It was just to be sure, because otherwise I have
no idea why setting the vscroll using the above code, does not
work here.
I have tried with the Fedora system Emacs 28.3, and with my own
build of Emacs 29.1 (with PGTK, but I guess that is the default now
on Emacs 29). Also, I have tried on wayland and x-org. But the code
does not set the vscroll when starting from emacs -Q, while, strangely
it works when I load my configuration.
Anyway, I guess I have to investigate some more then.
Thanks a lot for checking!