Subject: bug#3956: Happens on Windows too

From: Eli Zaretskii <[email-masked]>
Date: Sun, 22 Aug 2021 09:06:05 +0300

> From: Ville-Markus Yli-Suutala <[email-masked]>
> Date: Sat, 21 Aug 2021 23:40:48 +0300
> 
> Height of the scrollbar thumb changes while scrolling on Windows too.

That's how the scroll-bar thumb was designed to work in Emacs.