Bob JonkmanI dealt with a site that used Javascript to create the login fields, and also to stop copy'n'paste the password. Turn off Javascript to copy'n'paste, then the fields don't work.
I got around it by making my password manager #KeepassXC autofill the login fields. I'm guessing it stuffs password into the keyboard buffer rather than using the paste buffer.
Edit: add @keepassxc