This is the primary reason I don't use OneNote. I switch between notebooks frequently. The current setup takes up tones of space and also discourages browsing your notes... the opposite of what a good note taking program should promote.
Why not just a tree system, like a file browser? You could collapse and expand notebooks, section groups, sections and everything and see all the notes listed right under each section. Would be very accessible and save space as you could get rid of the top
navigation bar altogether and just have a single side nav.
Also the current way of having two separate ways of establishing note groupings is confusing. It took my Dad 15 minutes to figure out how it worked. It took me a couple, but that is a long time for me, as I would qualify myself as tech savvy. And there are
actually 3 different components to note navigation currently: the notebook drop down, the sections, and the note list. Jumping inside a Section Group removes all context of where you actually are because there is no bread crumb or other indicator of your browsing
path to get to that group. The current section navigation is a tree structure, but strung out horizontally. Why would anyone do this? *shake my head*.