Thanks for the link. As arcanist is the php-based client to speak to Phab, does this mean it's a user setting only and is fully respected by Phab, meaning it's something set on first publishing of a diff and kept in that setting? Is this also possible when submitting a diff via the web interface?

What's funny is that mutability didn't exist for Mercurial support because when it was written Mercurial had no official mutability feature.

And I can only suppose mutability being default for git support is because of Facebook's dev style. It's wrong to assume that those who want merge commits for the final merge onto master do not rebase locally before sending a pull request. Rebase is a standard tool for working on your branch (aka patch set aka patch queue) and it's normal to rewrite it upon each revision of the branch sent for review. Does phab support multiple revisions of a branch in one diff or does it require separate diff tickets? This is something that Gerrit got right, allowing one to see the diff between revisions of a branch (patchset).