I found this:
https://github.com/linuxmint/nemo-extensions/
but here is not mint, is ubuntu 18.04
This extension code is in python.
It is the only reason that I still open nautilus for: to compare files easily...
this didnt work for me: How to install nemo extensions?
I tried to download and ./build nemo-compare on ubuntu 18.04, but it requires nemo-3.8 and here is 3.6, found no updates to it.
there are some other files for older ubuntu, but I see no reason to them not work on 18.04, they are at:
https://launchpad.net/~noobslab/+archive/ubuntu/nemo3/+packages
I just dont know what to do with them...
If extensions could be installed for my user only and not at /usr it would be easier, but I found no info where it should be placed.
I tried temporarily copy them to /usr/share/nemo-compare and create a symlink for the main file at /usr/share/nemo-python/extensions/ but when restarting nemo, it didnt detect the extension... (so I removed all these tmp changes)