Presently, rather than centering the UI element like a lightbox however, I find it best to keep it relative to their mouse, so that they don't have to traverse halfway across the screen with their mouse to interact with the interface if they've clicked something (for instance) on the bottom left.

The problem however should be apparent: if they click something towards the bottom of the screen, we need to adjust the css top/left properties so that the UI remains within the viewable area and doesn't run beyond the viewport, if possible. I've seen this done tons of times with tooltips.

I've thrown some basic awareness together using $(window).height() / width(), but if anyone could point me to a mature algorithm for harvesting the appropriate offset, I'd love to see how this is best done.