getCellIndices({height, width, x, y}: SizeAndPositionInfo): Array<number> {
    const indices = {};

    this.getSections({height, width, x, y}).forEach(section =>
      section.getCellIndices().forEach(index => {
        indices[index] = index;
      }),
    );

    // Object keys are strings; this function returns numbers
    return Object.keys(indices).map(index => indices[index]);
  }
Gets all cell indices contained in the specified region.
A region may encompass 1 or more Sections.