1. Detect the first time a site (domain? URL?) attempts to copy something that's not visible. Algorithm TBD but we can start by warning too frequently.
2. Ask the user if this is a trusted site and deny, allow once, allow always. Users presumably select allow-always for apps like Google sheets.
3. (advanced) detect if enough people over long enough time select allow-always and then allow users to go with the herd. I'm talking 10+mm users not 10k, i.e. hard to cheat.
4. (Advanced) option to see what's in the proposed copy buffer.
(Obviously this all assumes that publishing sites have web security measures in place e.g. no raw HTML...)