Support adjustable left-handside sidebar width (on mobile)

I use the Amplenote app on a book-style foldable. Currently, the side-bar on the left takes up ca. 45% of the screen's width, which is way too much, there is a lot of wasted space. The only other option I have, is to completely hide it, which I don't want.

I would like to suggest for the split to be freely adjustable by the user being able to move the vertical 'separator line' on the horizontal axis. Thank you in advance for your votes and consideration