Great plus for adding the undo feature. However, I would suggest to move the button in mobile app (Android, I don't know about iOS). It is too close to the "back" button. In fact now there are 2 arrows directly next to each other, which can be confused easily.
- Move undo button to the right (between notebook dropdown and ... menu).
- you could even make undo and redo visible all times, then the confusion between "undo" and "back" would be minimal.
- Or even put undo function into the ... menu (since normally you don't need this feature so often)