Popup properties

I have created a few custom popups on my site, and I am trying to get them to scroll with the page like the default xenforo popups.

Does anyone know what I should do to give my popups this property?
So pretty much all of the popups on my site require you to register to see them. I sent you a private message with test account details since registering on numerous different sites can be a pain.
Isn't it just position: fixed which is required?
DUH. haha thanks, this was changed a long time ago in the template, and I overlooked the fact that I had position:absolute thinking it was some other problem. That's usually how it is though; you overlook what's right in front of you.