Essentially you’ll need to create two equal height columns — one for the fixed element and the other for the content — and then set the fixed columns content to sticky so it stops scrolling once it hits the top of the page.
From what I can tell via the Web Inspector you’ll need to adjust your structure a bit before you’ll be able to get things working like you’re hoping, so if you can shoot over your read-only link I can take a peek at the project in the Designer and walk you through the specific changes you need.