Old problem, still current in Ubuntu 18.10.
libinput and evdev are the main keywords to look for in duckduckgo if you want to resolve the middle mouse button issue. Some suggestions for a solution in the following.
You phrase the question quite specific ... I guess this is about Unix style copy paste in the terminal, right ?!?
Please have a look here:
and very proper:
How do I enable Middle mouse button emulation in 12.04 LTS?
It depends a lot on the Window manager. In XFCE different things work (because things work different) than in Gnome or Unity.
The confusion in config methods makes life hard to predict.
It may help to invoke the man evdev for modern systems.
I could post example configs (my Logitech Marble Mouse trackball actually does work perfect in XFCE4: Active middle mouse button emulation copy-paste and scrolling with the right little button). Tell me which one you'd like to see.
Just try all config advice to be found in the askubuntu etc., at least one solution will work.
Same idea, strange phrasing as well:
Chrome/Chromium middle mouse button for scroll Linux, Mac
You know this one?
Credits to whom it deserves: It looks to me like developers listened and enabled the middle mouse button emulation in Ubuntu 18.10 by default.
Thank you smart guys, this significantly reduces the stupid pain points in modern distros.