This blog post describes an historical feature (of Overleaf v1) which, at the time of writing this note, is scheduled to be replaced by improved functionality in Overleaf v2 (due for release on 4th September 2018). Consequently, this article is now of historical interest only.
Last month saw the release of two enhancements to the Overleaf editor: the first to help with document review, the second to improve the auto-complete functionality when inserting citations from your bibliography.