I'm having the same problem. The left panel with the tree view of all my notebooks has disappeared and I can't, for the life of me, figure out how to get it back. F10 does nothing and I don't have the option for Left Panel under View. I'm on a Mac using Version 7.14 (458244 Direct)
It should not be this hard to find! Please help! I went through all the options and none of them work to bring back the Left panel
I suggest adding some sort of button that would toggle the Notebook Tree view on and off.