Closed Unassigned: Modal child window does not prevent user from tabbing to items behind the window container [21062]

I have a Modal ChildWindow inside a WindowCotainer with ModalBackgroundBrush.After I Show the ChildWindow using the Show method ,although the WindowContainer prevents the user from directly interacting with the elements behind it , the user can use the tab or use arrow keys to interact with the elements.

How to prevent the user from tabbing to other elements behind the ChildWindow ?
Comments: Dupplicate of issue https://wpftoolkit.codeplex.com/workitem/17859