If you hold the mouse down and drag off-canvas to the left or bottom sides, the painting cursor shoots up to y = 0 and leaves a streak.

Thanks for the bug report. I've saved it in the GitHub issue tracker. Pretty weird behavior, definitely fixable.

Where does JSX + Coffeescript integration exist now?

I wrote a JSX transformer for Coffeescript, it's quite stable now:

https://github.com/jsdf/coffee-react-transform

I used this for a small app (drawplz.com) and liked it quite a bit. Thanks for your work.