Nice! and indeed needed...
How do you handle scroll areas? can one column be independently scrolling? that was my main pain point with JQueryUI draggable...
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When the mouse pointer gets close to the bottom of the viewport while dragging an element we scroll the document. Columns are not scrolled on their own, since the whole grid is a "block".
We built it because jQuery sortables were limiting us to use separate columns, which couldn't handle "wide" (more than 1 column in width) elements.
Note that your "Fork me on github" banner points to http://github.com/you, which shows me MY profile page :)