I made a custom password manager a while back [1] where I used the MutationObserver API [2] to detect when a field is unhidden or newly added. I mainly did it to deal with iClouds login flow. The performance of that could probably be improved by figuring out the container of the login and only attaching the observer to that. But the whole thing was never really finished and I eventually forgot about it.

[1] https://github.com/schroffl/browser-password [2] https://developer.mozilla.org/en-US/docs/Web/API/MutationObs...