Down state overlays, not replaces, Normal
I'm using three images for a button's Normal, Hover and Down states. But the button's Down state (which is actually a picture of the button pushed down) appears overlayed on top of the Normal state instead of replacing it.
If instead of using the built-in Down state, I put the "button down" image on a new state and add a trigger to show it, it works properly (or at least how I think is properly).
Is this just the way the Down state works, or is there something I've missed?