There's lots of discussion there. We went with a design that leans on getting out of your way and recovers some screen real-estate. We do recognize the one additional click required though. And we are actively listening to feedback and will continue to iterate.
We're working hard to improve usability and navigation in general. In addition to this sidebar change, we've made many nav updates: https://gitlab.com/gitlab-org/gitlab-ce/issues/26348.