This lightbox technique uses the new "Pin to Browser" feature as seen in 6.5, which keeps the lightbox centered no matter where you scroll.
One of our users, Ulrich, sent us this file to use as an example and it's awesome! The idea is that you create a dynamic panel which contains a grey background, with a lightbox ontop. You pin this whole dynamic panel to the browser, and when you want to show it, the lightbox is always centered.
In the Dynamic Panel Manager > select the background state > right click the lightbox dynamic panel in the working area > Edit Dynamic Panel > Pin to Browser.
In the Pin to Browser area you define where you want that dynamic panel to appear on the page. This is the same functionality as Page Align that appears in the Page Formatting panel. Typically you would set the Page Align and the Background dynamic panel the same. In my example I don't need to set the Background dynamic panel because it inherits the page formatting.