Thanks, Aaron and Sandy, for your replies. Aaron, you're suggesting exactly what I suggested as option (2) -- namely, that I keep track of which popup is open (which in fact I was already doing, in order to close it when the Mask was clicked), and close it upon opening a new popup. And in my case, I would also close it upon the user clicking one of the other buttons that initiates an AJAX request. (Don't want any popups open when the AJAX happens.)
Sandy, your approach of cutting holes in the mask will, I believe, not result in the mask getting the onclick event. I was looking for a way that the mask could receive the event, AND the underlying object (button) could ALSO receive the event. Also, I'm a bit nervous about the advanced CSS; I need this to work in browsers going back to IE 7. I ended up implementing Aaron's approach (== my option (2)); forget the mask, and just close the current pop-up in the cases where a user clicks a button to open a different pop-up, or to initiate an AJAX request.
