When you add content to a DropDownWindow, the DropDownWindow becomes the logical parent to the content. Similarly, the DropDownWindow content is considered to be the logical child of the DropDownWindow. The child content is not added to the visual tree that contains the DropDownWindow 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.

And when IsOpen is set to true, its content is added to the host Popup control, which becomes the logical child of this DropDownWindow at the same time.