Scrolling by clicking on steppers unbearably slow

A change was introduced on GTK+3 that makes scrolling by clicking on scrollbar steppers unusable, at a rate of about just 1 line per second, as reported by some users on multiple channels:

* https://askubuntu.com/questions/732796/ubuntu-15-10-slow-stepper-scrolling-in-terminal
* https://mail.gnome.org/archives/gnome-list/2015-October/msg00006.html
* https://lists.ubuntu.com/archives/xubuntu-users/2017-November/010358.html

Currently, the only way to scroll by using the steppers is by RIGHT-CLICKING on them, which is non-standard behaviour, which could be considered as an accessibility issue, as it is only documented in the GTK+ changelog:

https://abi-laboratory.pro/tracker/changelog/gtk+/3.18.2/log.html
* GtkScrolledWindow
 - Keep scrollbars below treeview headers
 - Switch the roles of secondary and middle click on scrollbar steppers
   - Primary click starts low-speed autoscrolling
   - Secondary click start high-speed autoscrolling
   - Middle click scrolls to the end
 - Tweak button bindings on scrollbars (and scales)
   - Primary click warps to the location
   - Primary click with Shift jumps by pages
   - Secondary click starts variable-speed autoscrolling

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: libgtk-3-0 3.22.24-0ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
ApportVersion: 2.20.7-0ubuntu3.1
Architecture: amd64
CurrentDesktop: XFCE
Date: Fri Nov 10 17:22:08 2017
InstallationDate: Installed on 2017-04-23 (201 days ago)
InstallationMedia: Xubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: gtk+3.0
UpgradeStatus: Upgraded to artful on 2017-10-28 (13 days ago)