showAt(opener, openerWidth) {
      this.selectWidth = openerWidth;
      this.respPopover.open = true;
      this.respPopover.opener = opener;
      this.hasValueState = !!opener.hasValueState;
      this.hasValueStateSlot = opener.valueStateMessageText.length > 0;
      this.valueStateText = opener.valueStateText;
      this.valueStateMessageText = opener.valueStateMessageText;
      this.valueState = opener.valueState;
      this._headerTitleText = opener._headerTitleText;
    }
Shows the dropdown at the given element.