Prevent context menu from being overlapped by code panel (Fixes #581)

I just learned about this project today but I've already started using it and I gotta say, it's definitely the simplest web-based IDE I've seen yet! So I figured I'd give a little bit (one line's worth) back :)

I was able to fix the context menu issue (#581) just by changing its position to "fixed."

Thanks for such an awesome tool!