I second this. In every other spreadsheet software I've ever used (Excel, Google Sheets, LibreOffice Calc etc.) holding the Shift key and using the arrow keys is a common way to select multiple contiguous cells without having to use the mouse. That does appear to happen in Proton Sheets, but in addition to that, it begins editing the first cell in the selection which deletes the current contents. This is a very frustrating UI choice that I would advocate strongly for switching to match the common convention used in other software.
I second this. In every other spreadsheet software I've ever used (Excel, Google Sheets, LibreOffice Calc etc.) holding the Shift key and using the arrow keys is a common way to select multiple contiguous cells without having to use the mouse. That does appear to happen in Proton Sheets, but in addition to that, it begins editing the first cell in the selection which deletes the current contents. This is a very frustrating UI choice that I would advocate strongly for switching to match the common convention used in other software.