What is it with services now separating username and password fields?

https://lemm.ee/post/10130956

What is it with services now separating username and password fields? - lemm.ee

Back in the old times, on the sites I log in regularly, my browser filled in both username and password. I clicked “Log in” once, and I was set to go. But no more. Now it’s all first a username, then a password. From what I saw, Apple started this many years ago, but now this bother really spread. And it’s not like I can just double-click on the same screen area, oh no. Animations make sure that I have to wait several hundred milliseconds before the password field is there, and depending on the site, I even have to select from my browser, which login I want to use, twice! Why, oh why? All my screens are really big enough to display 2 text fields. What are arguments for this behavior? I don’t see any.

A lot of services these days support multiple forms of authentication. Did you sign up with a separate password? Did you use Google or Facebook auth? Is this a corporate account where auth is via their SSO? They don’t even know if or whether they should ask for your password until they know who you are.
That’s the best explanation I heard so far.

As someone who just built one of these, that is the exact reason we did it.

It would be cool if users just remembered which service they used to sign in, but they often don’t, so this is the next best thing. Tell us your email, we look up which service you used, then send you to that service to complete the login.

Pro tip: leave the password field on the site but make it invisible. So when I am using my password manager to fill in the username, the password field will be filled out too. And I don’t have to use my password manager twice for one login.
1Password actually is really good at handling these two step login screens, for me it always autofills the password correctly
1Password is great, I just switched to it recently after the LastPass kerfuffle and the UX is lightyears better