So this interpret the sidebar as a temporary UI element (like a popover window), but for me I'd rather interpret the sidebar as a constant element (like the sidebar of Windows Explorer). And indeed I'm using Firefox with Tree Style Tabs or Sidebery to have an explorer-like tree view of my browsing session; it makes more sense for me to have this as a permanent fixture.

With that in mind, I would think this kind of preference should be under the control of the browser, not of an individual website.