WPF window control

I recently stumbled upon a WPF docking control from ActiPro.

They created some sort of WindowControl that is a control, which mimics a window, and thereby gives the ability to have floating windows and MDI in an xbap application.
I have a hard time grasping how they did this. It's a regular control, but can float outside of the window that contains it. As far as I can tell, they did not use user32.dll for this.

Anyone got any ideas? I'm scratching my hair thin.

With some further digging, I can see that they do in fact use user32, it's hidden in an internal class, but the whole thing is obfuscated beyond understanding