Fossil

    },

    /**
       Registers an event listener for this object's custom events.
       The callback gets a CustomEvent object with a 'detail'
       propertly holding any tab-related state for the event. The events
       are:

       - 'before-switch-from' is emitted immediately before a new tab
       is switched away from. detail = the tab element being switched
       away from.

       - 'before-switch-to' is emitted immediately before a new tab is
       switched to.  detail = the tab element.

       - 'after-switch-to' is emitted immediately after a new tab is
       switched to.  detail = the tab element.

       Any exceptions thrown by listeners are caught and ignored, to
       avoid that they knock the tab state out of sync.

       Returns this object.
    */
    addEventListener: function(eventName, callback){
      this.e.container.addEventListener(eventName, callback, false);
      return this;
    },

    /**
       If the given DOM element, unique selector, or integer (0-based
       tab number) is one of this object's tabs, the UI makes that tab
       the currently-visible one. Returns this object.
       the currently-visible one, firing any relevant events. Returns
       this object. If the argument is the current tab, this is a
       no-op, and no events are fired.
    */
    switchToTab: function(tab){
      tab = tabArg(tab,this);
      const self = this;
      if(tab===this._currentTab) return this;
      else if(this._currentTab){
        this._dispatchEvent('before-switch-from', this._currentTab);
      }
      delete this._currentTab;
      this.e.tabs.childNodes.forEach((e,ndx)=>{
        const btn = this.e.tabBar.childNodes[ndx];
        if(e===tab){
          if(D.hasClass(e,'selected')){
            return;
          }
          self._dispatchEvent('before-switch-to',tab);
          setVisible(e, true);
          this._currentTab = e;
          D.addClass(btn,'selected');
          self._dispatchEvent('after-switch-to',tab);
        }else{
          if(D.hasClass(e,'selected')){
            return;
          }
          setVisible(e, false);