Looks pretty cool! A couple quick notes:
The section edit + dropdown menu only appear on hover; this feels very odd on a tablet as you have to actively tap to make it appear (on iPad) or it may not work at all (on Windows 8.1). I like the idea of 'edit' being in the fixed header that's consistently visible instead...
On small screens like phones I've been seriously considering suggesting we show a fixed section header -- this sample article has relatively short sections but many are ....lllloooonnnngggg.... especially in really big articles. In a small window or on a smaller-screen tablet, those are going to easily spill larger than a screen height and it may feel odd to scroll up to find the current section-edit button.
Once you scroll past the menu items, the sidebar seems to be wasted space especially on a small window. Any plans to have it collapse if the window/screen size is smaller? Snapping a browser window to half the screen is *really easy* to do on current Windows and Linux versions, and seems like something I'd expect people doing research to do.
-- brion