Add a "View" button next to "Edit". This would
(i) allow you to view details of an invoice not otherwise shown (eg job, product codes) without risk of changing anything accidentally and
(ii) still work after the invoice is locked. Currently, if you want to view those details of a locked invoice, you need to use the nuclear option of going into Settings and unlocking everything for editing that is supposed to be locked. That is ridiculous and dangerous.