_getItemPosition(itemSize) {
    return getItemPosition({
      itemSize,
      positions: this.positions,
      gridSize: this.colWidth,
      total: this.cols,
      threshold: this.options.columnThreshold,
      buffer: this.options.buffer,
    });
  }
Determine the location of the next item, based on its size.
@param {{width: number, height: number}} itemSize Object with width and height.
@return {Point}
@private