Function XdgPositioner.setAnchor
set anchor rectangle anchor
Defines the anchor point for the anchor rectangle. The specified anchor is used derive an anchor point that the child surface will be positioned relative to. If a corner anchor is set (e.g. 'top_left' or 'bottom_right'), 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.