Translucent Dialog Boxes that don't show through each other

It would be real nice to have translucent dialog boxes that don't show through each other.

If two of the translucent dialog boxes overlap, in the area where they overlap only the one that most recently had the focus would show in the overlap area.

This would make the it possible to have non-overlapping dialog boxes that could be used while not blocking the view of the world, and when it is needed to overlap the dialog boxes, such as when you open the preferences, the one you are working in is legible, without all the stacked up translucent dialog boxes showing through.