In papers 3, when you had many articles open in the read pane, but needed to view more, you could open more articles and then navigate to those additional articles from a drop down menu that would appear (on the right of the app window). That way, the tabs in the read pane would remain readable. I have 16 articles open now (for a writing project), but the tabs only have 1 letter as a title, which isn't meaningful. So I end up clicking through the tabs to find the open article I want to look at. The drop down menu from Papers 3 allowed you to have more articles open in the read pane, and be able to quickly go to a particular article (as the tab was titled with the first author's last name, and year of pub).
(maybe I'm missing how to do this in the new papers??)