Adwaita: Avoid fine-tune scrollbars jumping

Fixes #3672
7 jobs for wip/jimmac/scrollbars-fine-tune-fix-gtk3 in 30 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1158644
debian-meson

00:04:16

passed #1158641
fedora-autotools

00:05:49

manual #1158642
allowed to fail manual
fedora-distcheck
passed #1158643
fedora-meson

00:14:00

manual #1158646
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #1158645
win32-ps
msys2-mingw64-meson

00:03:29

 
  Docs
passed #1158647
reference

00:16:23