code: func ChildAt(parent Control, x, y int) Control {
  	px, py := parent.Pos()
  	pw, ph := parent.Size()
  	if px > x || py > y || px+pw <= x || py+ph <= y {
  		return nil
  	}
  
  	if len(parent.Children()) == 0 {
  		return parent
  	}
  
  	var ctrl Control
  	ctrl = parent
  	for _, child := range parent.Children() {
  		if !child.Visible() {
  			continue
  		}
  
  		check := ChildAt(child, x, y)
  		if check != nil {
  			ctrl = check
  			break
  		}
  	}
  
  	return ctrl
  }
docs: // ChildAt returns the children of parent control that is at absolute
  // coordinates x, y. Returns nil if x, y are outside parent control and
  // returns parent if no child is at x, y