bug#42406: Mouse-wheel scrolling can be flickering

[Eli Zaretskii]

> From: martin rudalics <rudalics@gmx.at>
> Date: Wed, 16 Dec 2020 09:46:18 +0100
> Cc: Alan Third <alan@idiocy.org>, 42406@debbugs.gnu.org
> 
>  > Performance does not degrade when only the two calls of
>  > wset_update_mode_line you mentioned are commented out. However, with
>  > just commenting out these two instances, I still have a scroll
>  > performance problem when scrolling by using the scroll-bars. Could you
>  > also identify the instance concerning this?
> 
> I forgot how Emacs precisely handles that.  Maybe Eli can tell.

See scroll-bar-toolkit-scroll.  Basically, it calls
scroll-up/scroll-down.