Here's a screenshot of the page behind the scenes:
The most important regions are the "List of Customers", "List of Orders" and "Order Items", those are the regions that you see on the first screenshot. The other region "Manage Order Items" is a Modal Dialog that comes when you want to add an order item.
So almost natively you can build your Master-Detail-Detail, through their wizards.
I thought to mention this solution here as well, as although my first choice is to make simple and clean web applications, if you do want a lot of information on your screen (like in a master-detail-detail), and you like ExtJs (which is used behind the scenes), FOEX is probably one of the best choices you have.
APEX R&D is a partner of FOEX, so if you need some more info, don't hesitate to contact us.