Howdy folks...I just got a new work laptop with Windows 10 and OneNote 2016. I made the "Notebooks" menu on the left a little wider than it was (by default) and now cannot make the width narrower! The cursor changes to the + icon yet it will not drag left to reduce the width; I can only entirely unpin.
And I can now answer my own question...If you close OneNote and reopen it you can resize to make it narrow again. Crazy to me but there you go! Only thing I can think of is expanding it to a certain width did an inherent "snap to width" that couldn't be undone before closing and re-opening?