This started just a few days ago. I click a cell, and a cell down and to the right is selected. In one sheet, it's down 2 rows and over 1 column; In another sheet, it's down 5 and over 1 and another down 6 an dover 4. I cleared cache and returned to that sheet and now it's down 5 and over 3.
Also, in a workbook with multiple worksheets, the tabs at the bottom have vanished.
Google Sheets work perfectly in Firefox, so I guess I'll have to use Firefox, but it seems as though Google Sheets should work in Opera. Up until a few days ago, it did. I do have latest version of Opera (78.0.4093.147). Also latest version of MacOS.