Each time a user selects a Notebook from the sidebar ALL of the Tab heading show up in a list directly below the Notebook. I am sure this was intended as an alternate to using the actual tabs. In and of itself this is not an issue. However .... if the user selects a different Notebook, then its sub heading display below it as well. If someone (like myself) have even a handful of Notebooks and sub headings the Sidebar turns into a Long List that one has yo read through.
My suggestion is to simply add a feature that closes the subheadings automatically from any open Notebook, when a different Notebook gets selected. This would allow use of the subheading but not creating the endless long list to read through.
I hope you agree and find this idea helpful.