code: function handleHiddenChange (hidden, oldHidden) {
      if (!hidden && oldHidden) {
        positionDropdown();
  
        // Report in polite mode, because the screen reader should finish the default description of
        // the input element.
        reportMessages(true, ReportType.Count | ReportType.Selected);
  
        if (elements) {
          $mdUtil.disableScrollAround(elements.ul);
          enableWrapScroll = disableElementScrollEvents(angular.element(elements.wrap));
        }
      } else if (hidden && !oldHidden) {
        $mdUtil.enableScrolling();
  
        if (enableWrapScroll) {
          enableWrapScroll();
          enableWrapScroll = null;
        }
      }
    }
docs: event/change handlers 
  Handles changes to the `hidden` property.
  @param hidden
  @param oldHidden