Another stacked PR user here (Graphite) and this is a real pain point for us.

Since we started using Stacked PRs, this became a big challenge for us!

I would like to chime in here to voice our interest in this feature. Submitting pull requests from a personal fork of a github repo is a very common workflow that's actively encouraged by github themselves: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks

Not being able to see previews for these types of PRs is a challenge for open source projects that depend on external contributions from developers that usually do not have commit rights on the upstream git repository.

Is this a feature yet or possible with Preview Environments perhaps?

This feature should include previews when PRs are made between branches i.e. branch -> branch