PopupPositionerParametersAnchor Property
Defines the anchor point for the anchor rectangle.
The specified anchor is used derive an anchor point that the popup will be positioned
relative to. If a corner anchor is set (e.g. 'TopLeft' or 'BottomRight'), the anchor
point will be at the specified corner; otherwise, the derived anchor point will be
centered on the specified edge, or in the center of the anchor rectangle if no edge is
specified.