Background sliding with Accordions?

Hi all. I’m having a problem with my theme (in development)… When you click Comments (bottom-right), a comments tray is supposed to slide out using an Ajax accordion, but the page background slides with it.

It’s as if the background is anchored to the BOTTOM of the page, rather than the top. This is likely the cause of great slow-down (laggy frame-rate) while the accordion is sliding, since there are many partially transparent layers being re-rendered while the background slides behind everything, so it MUST be fixed if at all possible. (also because the sliding background just looks funny/distracting) Here’s a link:
https://dlspector.net/omega/?p=1

Any ideas?