In the current design the sidebar will automatically expand when your browser window is wide enough, at which point the side bar will stay expanded or collapsed (it will not exhibit the behavior you described).
When the window is narrower the sidebar is automatically collapsed. In that mode when the sidebar is expanded the sidebar floats above the content of the page. We generally don't want the sidebar pinned obscuring the content.
For quick access to the sidebar, consider using the '[' keyboard shortcut.
I've just check this and even at full width (1920px) on my browser (Chrome 35.0.1916.153 m) the side bar is not automatically expanded and still opens and closes when you click on the large empty grey area.
Maybe I'm reporting a bug as this doesn't sound like the expected behaviour you describe above?
It sounds like it's possibly a bug. The sidebar is supposed to auto expand and 1280px. When you have your window expanded wider than 1280px and expand the sidebar, does it obscure the repository content like this (screen shot slightly narrower than 1280px):
or do you see it like this (slightly wider than 1280px):