> It won't be a red flag for people who often see auto-complete not working for legitimate websites. The usual cause is legitimate websites not working instead of actual phishing attempts.
Yeah, that's true, I hit this all the time with 1Password+Firefox+Linux (fun combo).
Just copying-pasting the username+password because it doesn't show up is the wrong approach. It gives you a chance to pause and reflect, since it isn't working, so in that case you lookup if it's actually the right domain, and if it is, add it to the allowed domains so it works fine in the future.
Maybe best would be if password managers defaulted to not showing a "copy" thing at all for browser logins, and not letting users select the password, instead prompting them to rely on the autofill, and fix the domains if the autofill doesn't work.
Half the reason I use password manager in the first place is specifically for this issue, the other half is because I'm lazy and don't like typing. It's really weird to hear people using password managers yet do the old copy-paste dance anyways.