I believe this is because you do not have push permissions to a contributor's forked repo by default (even with maintainers can edit). Doing it in the UI bypasses this, of course.

You should add their repo as a remote:
git remote add <contrib-fork> <https://github.com/contributor/repo.git>

You can then fetch: git fetch <contrib-fork>, check out: git checkout -b <contrib-branch> <contrib-fork>/<contrib-branch> , make changes and push to their PR branch: git push <contrib-fork> <contrib-branch>.

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I believe this is because you do not have push permissions to a contributor's forked repo by default (even with maintainers can edit). Doing it in the UI bypasses this, of course.

You should add their repo as a remote: git remote add <contrib-fork> <https://github.com/contributor/repo.git>

You can then fetch: git fetch <contrib-fork>, check out: git checkout -b <contrib-branch> <contrib-fork>/<contrib-branch> , make changes and push to their PR branch: git push <contrib-fork> <contrib-branch>.

This doesn't work. I still get the same error

$ git push <contrib-fork> <contrib-branch>
remote: Write access to repository not granted.
fatal: unable to access 'https://github.com/user/repo.git/': The requested URL returned error: 403

I have write access for the main repository. The forked repository is on a user account. The user selected Maintainers are allowed to edit this pull request. I can edit through github.com one file at a time but nowhere else.

Every time I come back to this issue, I am faced with the same problems.

Try using an SSH repo URL instead of an HTTPS URL.