The RapidFindReplacePopup control can be opened initially in various places on the Window. The user can drag the popup by the title bar, and resize it with the thumb grip.
<RapidFindReplace:RapidFindReplacePopupControl DockingPosition="MiddleCenter"/>
DockingPosition can be set to TopLeft, TopCenter, TopRight, MiddleLeft, MiddleCenter, MiddleRight, BottomLeft, BottomCenter, or BottomRight.