_handlePageDownFlat() {
      if (this._skipItemsSize === null) {
        // Move the focus to the very bottom (as End).
        this._currentIndex = this._getItems().length - 1;
        return;
      }
      const currentToEndRange = this._getItems().length - this._currentIndex - 1;
      if (currentToEndRange > this._skipItemsSize) {
        // When there are more than "skipItemsSize" number of items until the bottom,
        // move the focus down/right with the predefined number.
        this._currentIndex += this._skipItemsSize;
      } else {
        // Otherwise, move the focus to the very bottom (as End).
        this._currentIndex = this._getItems().length - 1;
      }
    }
Handles PAGE_DOWN in a flat list-like structure, both vertically and horizontally.