Floating popup vs Split Container

Hi,

My application uses dojo tabs and i have a situation where in i have a split container and now i have to show a floating div to which i have given a z-index. I want that div to be rendered always in the center of my browser window. But while i am trying it, it is always coming on one side of the Split Container.

How can i ensure that it opens in the center only?

Neha