If you've ever wanted to make minor changes before merging a pull request, now you can. When a user opens a pull request from a fork, they'll be given a new option that allows the upstream repository contributors to collaborate with them on their new branch.
Simply clone the contributor's fork and, if you've been granted permission, you'll be able to push changes to the head branch on the open pull request, even if you don't normally have permission to push to the fork.
Pull requests created before the option was available will default to not allowing collaboration, but contributors can also enable or revoke permission, by using the new checkbox in the pull request sidebar.
Learn more about collaborating with forks in our help guide.