Collapsible sidebar experiments

I don't need a collapsible sidebar, since I heavily rely on tags for organization. I don't have a large pile of items, I organized them.

However, I see a use case on screen space restricted systems. If you intend to also migrate the v8 client to mobile. Look at the category bars in Microsoft's OneNote App (the app, not the desktop version). If the app runs on a mobile device, the sidebar(s) are dynamically scrolled to the left out of screen if you navigate to the right to view your documents. And if you move back to the left, your contents is dynamically scrolled to the right out of the screen to make space for the category bars.
This way the information you currently work on gets enough space to work comfortably.

If the app is run on a desktop system, either all sidebars are always visible, since there is enough screen space, or you collapse them manually, which leaves a tiny sidebar with a button to expand the sidebar again, a search button, and a history button - the standard mobile view.