I don't think it can be done today, with the remote domain (gmail.com) controlling all the JS on the page. But maybe future hardened browsers could attach a tex

I don't think it can be done today, with the remote domain (gmail.com) controlling all the JS on the page. But maybe future hardened browsers could attach a textinput box directly to a sandboxed service worker not communicable with the rest of the JS code? The worker could be updated at a different cadence than the rest of the client, and be audited separately, and with the script contents hashed? We rely on browser sandboxing of different domains today, why not extend it to different parts of the same page? Its probably a hard problem still.