Multiple column drop-down menu

I'm using the superfish module to create nice drop-down menus. I'm running into the problem, though, of my menu panels running off the bottom of the screen. Since the menu is on a fixed header, that content is inaccessible.

So I'd like to set up the menu so that each drop-down panel has a maximum height, and once that height is reached the panel expands into a new column. Any idea how I could do that? Is there another menu module that would make this easier?

Thanks!