Not the index of col in spreadsheet, but index of first column in current viewport. e.g. if col start C at current viewport after freeze, and scroll value is zero, startColumn is 0. e.g. if scrolled about 2 columns, now left is E, then sheetViewStartColumn is 2.
The index of row in spreadsheet. e.g. if row start 10 at current viewport after freeze, and scroll value is zero, startRow is 0. e.g. if scrolled about 2 rows, now top is 12, then sheetViewStartRow is 2.