Since in Silveilght 4 there is no WindowControl, Victor Gaudioso decided to make his own version that can be dragged around, expanded and collapsed.
Source: Victor Gaudioso's Silverlight Blog
If you watch the video you will wee that after I drag, expand and collapse the window a few times I get a JavaScript error marked by an exclaimation point in the bottom left of the browser window. Shawn Wildermuth speculates that it is because the Behavior get’s confused about who its parent object is. The way I fixed it was to wrap the Expanded grid into another Grid and put the MouseDrageElementBehavior on the parent Grid. I don’t show how to do this in the video but it is in my source code (ie., the source code has no JavaScript errorrs).