Don't waste so much space on the left sidebar

Please, please, please, don't waste so much space on the left sidebar. Keep the left-sidebar as collapsible/slide-out/pop-over/drop-down. Don't have it visible 100% of the time. We care about questions and answers. We want to see Q & A. We don't want to waste a bunch of space on those controls.

Update:
A per-site user preference has been added which allows you to "Hide left navigation" on a per-site basis. You will need to go to your local preferences on each site and check the "Hide left navigation" preference. There's no stock way to set this preference network-wide. While I don't think this solves the issue, it does help.

Userscript to move the left-sidebar into the topbar network-wide ( Stack Apps ) ( install ) ( GitHub )

Because I really don't like the left-sidebar, I created Left-sidebar in the Topbar, which moves the left-sidebar into the topbar on all SE sites which have the left-sidebar, regardless of the preference set on each individual site. Installing this userscript is equivalent to having selecting an SE network-wide preference to "Hide left navigation". You will not need to set the preference on each individual site.