New Post: IsActive not set when document is undocked

I've been following the MVVM sample whilst building my application and everything works great except for one issue that the IsActive property is not set when i click into the content area of a document. This works fine if the document is docked. For an undocked document IsActive is only set if you click in the title bar area.

I'm using a modified version of the Aero Theme (but haven't changed anything relating to undocked/floating windows) and can't seem to find where i'm going wrong

Any help would be much appreciated