On Github, I've run across a pull request which it implements a feature I like but feel isn't quite intuitive and could be better. The maintainers of the original repo have so far declined to accept it (or explain why). It was requested 3 months ago so I suspect they won't take it without changes.
I'd like to improve upon this contributor's code but don't know the etiquette: should I fork this fork? Or do I fork the main repo and add the other fork as another upstream source? If the latter, how do ensure credit is given where due? What implications does either option have for pull requests?
EDIT: While I appreciate hearing opinions and did ask about etiquette, I'm more interested in knowing about whether there is any technical or pragmatic reason to fork off the main project and add the creator of the 'cool feature' PR as an upstream versus just forking the branch of the 'cool feature' PR.
Based on jan-hudec's answer I'm inclined to fork main + add new upstream. I suspect it'd be easier to keep my fork up-to-date and not reliant on this other contributor to incorporate upstream changes.