To adhere to CONTRIBUTING, you will probably want to squash the two commits into one

Hey @jonringer, that isn't quite the case. CONTRIBUTING.md doesn't state anything on it but does refer to the nixpkgs-manual, and the manual states

Make commits of logical units.

I believe this is what I've done.

I always took it as atomic "package changes" that all correlated to a given change. This way, all changes can easily be reverted if needed.

I thought that may be the case too, but then I don't see why not just let the commits be while in the PR state and then "squash and merge"

In that specific case I think it was fine to just push changes until it works and use the squash button at the end. Having separate commits helped me with the review as well.

yep git rebase -i ftw. A better pr/diff viewer also helps a lot. I'm partial to reviewable.io myself since it doesn't try to take over the development model like gerrit does.