I have installed Tweaks and turned Middle Click Paste off. That doesn't help.
I have also followed all of the recommendations in these posts:
How do I disable middle mouse button click paste?
However, I can't get it disabled. It's a very annoying behaviour, in my opinion, and I can't see why it's so hard to get rid of it. Seems like a huge bug.
$ xinput list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ Elan Touchpad id=12 [slave pointer (2)]
⎜ ↳ Elan TrackPoint id=13 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Video Bus id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Power Button id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ Integrated Camera: Integrated C id=10 [slave keyboard (3)]
↳ Integrated Camera: Integrated I id=11 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=14 [slave keyboard (3)]
↳ ThinkPad Extra Buttons id=15 [slave keyboard (3)]