Closing a web popup that is still loading

I am using web popups to display pages that often load very slowly. If you wait for the popups to load before they get canceled, all works as expected. If you cancel one before the load is completed, it disappears, but a new webpopup cannot be created until (it seems) the original one would have finished loading. Is there any way to truly destroy a webpopup that is still loading? Or, as a far from ideal solution, can I capture some kind of event that indicates it loaded/failed to load so the back button can be disabled while it loads?
[import]uid: 28659 topic_id: 7477 reply_id: 307477[/import]