code: function _LightDismissableLayer_hiding(client /*: ILightDismissable */) {
                      var index = this._clients.indexOf(client);
                      if (index !== -1) {
                          this._clients[index]._focusable = false;
                          this._activateTopFocusableClientIfNeeded();
                      }
                  }
docs: Dismissables should call this at the start of their exit animation. A "hiding", dismissable will still be rendered with the proper z-index but it will no longer be given focus. Also, focus is synchronously moved out of this dismissable.