It appears there is more content to the right and you are simply scrolling the page (moving the view opposite to direction of mouse movement) and once you hit the edge it seems to invalidate the drag or something similar.
This is just from playing with it; I haven't had time to check the source yet.
Re:Daux.io, a simple documentation generator
by justinwars on Sunday July 14, 2013 @11:57PM
(#6043634)
I have heard some people having issues with horizontal scrolling, I am still trying to get to the bottom of this.
This _exact_ same effect happens on Outlook.com if you want to take a look at reproducing it there as well.