If we want to (eventually) make it possible to resize the sidebar, then having its width indirectly affect the toolbar is going to wreck havoc on the media queries used to show/hide buttons in the main toolbar (since many of them depend on the toolbar state, and thus its width).
Updating all of the media queries dynamically with JavaScript seems like a non-starter, given that it'd cause *very* messy code. It thus seem to me that we'd need to fix the position of the sidebar, to have any hope of (in the short term) addressing issue 2072.
Hence, I'm suggesting that the we always layout the sidebar in a consistent vertical position, and only animate the `viewerContainer` rather than the entire `mainContainer`.
Fixes 4052.
Fixes bug 850591.