code: func (v *ViewPort) ValidateViewY() {
  	if v.viewy >= v.limy-v.height {
  		v.viewy = (v.limy - v.height)
  	}
  	if v.viewy < 0 {
  		v.viewy = 0
  	}
  }
docs: // ValidateViewY ensures that the Y offset of the view port is limited so that
  // it cannot scroll away from the content.