[andrewaylett]

That's a controversy I'm not sure you necessarily realise you've stepped into :).

It's fairly common to consider working and PR branches to be "unpublished" from a mutability point of view: if I base my work on someone else's PR, I'm going to have to rebase when they rebase. Merging to `main` publishes the commit, at which point it's immutable.

Working with JJ, its default behaviour is to consider parents of a branch that's not owned by you to be immutable.