In UIElementExtensions, IsAncestorOf crashes when the element parameter is of type Hyperlink.

The xaml team recently reviewed the status of this issue and won't be addressing it in future. 

If you are using PopupExtensions for Silverlight to set Placement of the Popup and you zoom your browser, the Popup will not be on the correct position if the element in which you've defined the Popup has Margin set.