Direct Post - hpp-listener-loaded message may send too early to cause the event can't be caught

[feng-xiao]

Hi, I'm implementing checkout solution using Direct Post. I've registered for a 'message' event, but the handler never receive message for 'hpp-listener-loaded'. I checked the source code from hpp iframe, the message has been posted, so the issue is the message is posted too early. Because I do receive messages for resizing the iframe with the handler.

Much appreciated if you could investigate the issues.