constructor(props, context) {
    super(props, context);

    this._loadMoreRowsMemoizer = createCallbackMemoizer();

    this._onRowsRendered = this._onRowsRendered.bind(this);
    this._registerChild = this._registerChild.bind(this);
  }
Threshold at which to pre-fetch data.
A threshold X means that data will start loading when a user scrolls within X rows.
This value defaults to 15.