Comment 3 for bug 1986168

@Dennis Evangelista:

I'm using 'Ubuntu on Xorg' with following configuration (see below) in '/usr/share/X11/xorg.conf.d/40-libinput.conf' and it works now (after multiple trying) correct...

Section "InputClass"
        Identifier "Marble Mouse"
        MatchProduct "Logitech USB Trackball"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "ScrollMethod" "button"
        Option "ScrollButton" "8"
        Option "MiddleEmulation" "true"
EndSection

Aktivation of scrolling per 'holding the little button ans scrolling the ball':

gsettings set org.gnome.desktop.peripherals.trackball scroll-wheel-emulation-button 8

I hope it helps you and some other user :-)