When you add content to a Popup control, the Popup control becomes the logical parent to the content. Similarly, the Popup content is considered to be the logical child of the Popup. The child content is not added to the visual tree that contains the Popup control. Instead, the child content is rendered in a separate window that has its own visual tree when the IsOpen property is set to true.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)