> Try to rebase it and you're going to be manually looking at every non-conflicting change that ever happened on that branch, for no apparent reason

My "fix" is to do an interactive rebase of PR B on main and drop all of PR A's commits from PR B in the process.

I remember seeing a way to do this automatically, but it requires an option that I never remember. IMO this is kind of the issue with git: a lot of improved workflows sit behind some flags that most people never learn. Interactive rebases work for me because they are one primitive, always working in the same way.