_handlePageUpFlat() {
      if (this._skipItemsSize === null) {
        // Move the focus to the very top (as Home).
        this._currentIndex -= this._currentIndex;
        return;
      }
      if (this._currentIndex + 1 > this._skipItemsSize) {
        // When there are more than "skipItemsSize" number of items to the top,
        // move the focus up/left with the predefined number.
        this._currentIndex -= this._skipItemsSize;
      } else {
        // Otherwise, move the focus to the very top (as Home).
        this._currentIndex -= this._currentIndex;
      }
    }
Handles PAGE_UP in a flat list-like structure, both vertically and horizontally.