Up to 18.04 it was possible to install nautilus-compare to get a context menu that provide a compare later and compare to option. Now that I'm trying an upgrade to 20.04 I'm wondering how to do this in Focal. I assume it can be done using the fma-config-tool, but if someone has done it already, I'd like to avoid trying to invent an inferior wheel myself.
Update 20200522: the package is available in PPA and is installable by:
The package was removed from official Ubuntu 20.04 LTS repository.
But you can still download package from 19.10 manually and install it:
then restart Nautilus and use it as before.
The package was most likely removed from 20.04 because it depended on Python 2, which is also no longer available in Focal.
The code has been updated to support Python 3 though. I'm not sure if/when it will return to the main repo, but the updated package is available in Märt Põder's PPA.