I am developing an MDI-like interface similar to your demo...
It would be great if Telerik could create some "snap" functionality into Rad Window, similar to the window snap functionality availalble in Windows 7+. I would like to be able to drag a rad window to the side of it's container and then have some way of "snapping" it so that it occupies 100% height and 50% width, left or right. It would also be cool if you could drag the window to the top or bottom of its container and then snap it their as well 50% height, top or bottom and 100% width). I'm not sure why the Windows folks didn't include vertical snap, but maybe there's some technical issue.
Anyhow, allowing users to quickly size windowsby snapping them for side by side (or top/bottom) display would really be useful for many MDI type implementations.
I imagine the Rad Window already has client-side capabilities for detecting when one of the window edges has reached its container boundary. They key would be providing users with some sort of visual cue that they can "snap" the window to that particular boundary (left/right and, ideally, top/botom).
Of course, it would probably be necessary to provide some sort of toggle property so users could turn this functionality on/off, or perhaps an enumeration property... like SnapBehaviors="EnableSnapVertical, EnableSnapHorizontal, EnableSnapLeft... etc.)