Right now, we can only navigate between notebooks in its respective pane using up and down arrow keys.
Since the addition of sub-notebooks, the only way to navigate to such via keyboard input is to keep its parent notebook expanded. This makes the notebooks pane really cluttered at times. Apparently, the space bar is the current key to do this. I didn't realise that until it was mentioned, and there isn't much in the way of documentation to speak of regarding it either.
Please allow the use of left and right arrow keys to fold and expand notebooks. It should be simple enough.