1. Create a new note or open a note that's not in a notebook.
2. Click on the Notebook button.

Nothing happens and it's not immediately obvious what the purpose of the button is when you don't know what a notebook is. The button's tooltip says "Place this note into a notebook", so the users expects this (whatever it is) to have happened after he clicked the button, even without visual feedback. I think it would be better to show the dropdown when the Notebook button is clicked instead of showing it when the arrow next to it is clicked.

Other information: