Yes, this is it! The default behaviour of gitlab was actually rather unexpected to me in that it cleverly gives the pull request a version which persists after rebasing etc. But I actually expected that drop down to let me switch between the commits in the PR. We use a process of amending PRs using fixups and only rebasing at the end because it allows other developers to easily pull the branch. Being able to step through the commits would be the best of both worlds.