Not only would you not want to, but it wouldn't matter. If the elements set to fixed it will do nothing to set it's top and left properties.
Your snippet might work for setting the position, but for the other functions like centering the element in the window, and the fromX functions it must be absolute. I'd also like to add to it, offsetFromElement(e, x, y) offsetFromMouse(x, y) This would be a major convenience function for generating tooltip like items, or other contextual windows. Again I've run against this a few times in recent projects and it's a real pain to try out these interactions when it takes a lot of work.
