Repeat after me: Separating username and password fields on separate (fucking responsive) page WILL NOT INCREASE A FUCKING SECURITY IN ANY WAY! IT WILL JUST MAKE THE PASSWORD MANAGERS TO WORK WORSE AND THUS IT WILL FUCKING DECREASE THE SECURITY!!!
@ondrej One reasonable possibility for the pattern is systems that implement SSO login flows for *some* users, and they don't want an SSO-bound user to spend time worrying about inputting a password that may indeed not exist.
They need to evaluate the username to decide whether to prompt for a password at all.
Which password managers are causing you problems?