Thanks for the great works.
I downloaded 59315 and replace my project right now.
Anyway, even the DemoApp.SampleDockableContent.xaml can not make a preview XAML in VS2010.
The "Document outline" panel in VS shows each UI component underneath the DockableContent.
But strange thing is, entire preview shows nicely in MainWindow, the whole components are in together.
How can I make to show a design preview of each XAML DockableContent in VS?