docs: function () {
                  var constrainTo = this.params.constrainTo, domUtil = ariaUtilsDom;
                  if (!constrainTo || constrainTo === domUtil.VIEWPORT) {
                      this._boundary = constrainTo;
                      return;
                  }
                  if (ariaUtilsType.isString(constrainTo)) {
                      constrainTo = domUtil.getElementById(constrainTo);
  
                  }
                  if (constrainTo) {
                      this._boundary = ariaUtilsDom.getClientGeometry(constrainTo);
                      return;
                  }
                  this._boundary = null;
                  this.$logError(this.INVALID_ATTRIBUTE, ["constrainTo", "params"]);
  
              }