In other spreadsheet editors, such as Google Sheets, if you hold Ctrl and click on multiple cells to select them, you can click a selected cell again to unselect it. This is very helpful if you're multi selecting and accidentally select a cell, since it'll let you unselect it and still keep all your other cells selected.

I tried this in Proton Sheets, and clicking on an already selected cell (in multiselect) does nothing.

It's a very small feature, but a good quality of life by design. :)

Thank you!