Edge, combined with animation frame listeners to create a parallax effect while moving the camera (because it seemed overkill, and would make it hard to do the proper calculations, to move the front and back layers inside Edge itself)
We were impressed with how much Edge handled for us while building this, even though I had to hack around in the raw javascript source a few times to fix some repositoning issues... will be filing some bugreports soon once I've got them properly reproduced.