public Point Offset { get; set; }

For example if the anchor of the anchor rectangle is at (x, y), the popup has the gravity bottom|right, and the offset is (ox, oy), the calculated surface position will be (x + ox, y + oy). The offset position of the surface is the one used for constraint testing. See set_constraint_adjustment. An example use case is placing a popup menu on top of a user interface element, while aligning the user interface element of the parent surface with some user interface element placed somewhere in the popup.