func (f *File) adjustMergeCells(xlsx *xlsxWorksheet, dir adjustDirection, num, offset int) error {
	if xlsx.MergeCells == nil {
		return nil
	}

	// for i, areaData := range xlsx.MergeCells.Cells { // ---
	for i := 0; i < len(xlsx.MergeCells.Cells); i++ { // +++
		areaData := xlsx.MergeCells.Cells[i] // +++
		coordinates, err := f.areaRefToCoordinates(areaData.Ref)
		if err != nil {
			return err
		}
		x1, y1, x2, y2 := coordinates[0], coordinates[1], coordinates[2], coordinates[3]
		if dir == rows {
			if y1 == num && y2 == num && offset < 0 {
				f.deleteMergeCell(xlsx, i)
				i-- // +++
			}
			y1 = f.adjustMergeCellsHelper(y1, num, offset)
			y2 = f.adjustMergeCellsHelper(y2, num, offset)
		} else {
			if x1 == num && x2 == num && offset < 0 {
				f.deleteMergeCell(xlsx, i)
				i-- // +++
			}
			x1 = f.adjustMergeCellsHelper(x1, num, offset)
			x2 = f.adjustMergeCellsHelper(x2, num, offset)
		}
		if x1 == x2 && y1 == y2 {
			f.deleteMergeCell(xlsx, i)
			i-- // +++
		}
		if areaData.Ref, err = f.coordinatesToAreaRef([]int{x1, y1, x2, y2}); err != nil {
			return err
		}
	}
	return nil
}