stop(event) {
    if (!this.isStopped()) {
      this._stopping = true;

      // noinspection JSUnresolvedVariable
      if (!event.simulated) {
        event.stopPropagation();
      }

      return true;
    }

    return false;
  }
Stop an event from further propagation
@param {SyntheticKeyboardEvent} event Event to call stopPropagation() on
@returns {boolean} true if the event was stopped and false if it was already
         stopped