Hi Mark, the new source browser combines the overview and source tabs. The items that you typically found in the overview can now be seen in the new right-side navigation menu. We are still working to add additional features, so if your new users would like to revert to the old experience they can via Bitbucket Labs in the settings menu. (found by clicking on your avatar in the lower left corner)
let me know if you have additional questions. I am going to close this ticket but if you have any problems, please open another ticket with our support team and mention me directly on it.