code: func FrameExtentsGet(xu *xgbutil.XUtil,
  	win xproto.Window) (*FrameExtents, error) {
  
  	raw, err := xprop.PropValNums(xprop.GetProperty(xu, win,
  		"_NET_FRAME_EXTENTS"))
  	if err != nil {
  		return nil, err
  	}
  
  	return &FrameExtents{
  		Left:   int(raw[0]),
  		Right:  int(raw[1]),
  		Top:    int(raw[2]),
  		Bottom: int(raw[3]),
  	}, nil
  }
docs: // _NET_FRAME_EXTENTS get