Actually, I think it's some obscure interaction. Maybe you should bring it up with the developers and see what they say. I've looked into fixing it, and I can't find anything wrong with the way I'm doing it. Not only does each part have a z-index specifying the stacking order, but they are in the document in the order they should be stacked, which is the fallback plan if there are no z-indexes, according to the standard, IIRC.