getTotalSize(): number {
    const lastMeasuredCellSizeAndPosition = this.getSizeAndPositionOfLastMeasuredCell();
    const totalSizeOfMeasuredCells =
      lastMeasuredCellSizeAndPosition.offset +
      lastMeasuredCellSizeAndPosition.size;
    const numUnmeasuredCells = this._cellCount - this._lastMeasuredIndex - 1;
    const totalSizeOfUnmeasuredCells =
      numUnmeasuredCells * this._estimatedCellSize;
    return totalSizeOfMeasuredCells + totalSizeOfUnmeasuredCells;
  }
Total size of all cells being measured.
This value will be completely estimated initially.
As cells are measured, the estimate will be updated.