Ask HN: Why are sites now breaking login forms into stages (name then password)?

I'm really looking forward to an answer for us, if someone with deep and relevant knowledge is around. There are a couple of possibilities that come to mind:

1) Perhaps testing reveals that some users are pushed away by the complexity of being confronted with two fields at the same time, and these users are more likely to successfully login presented with only one field at a time.

2) Perhaps there is some actual good security reason for it.

3) Perhaps there is some bad security reason for it. First example, lots of sites appear to express a belief that password managers are evil, and that users must be forced by increasingly obstinate means to type each long detailed robust password one single character at a time. Maybe this is simply an extension of that somehow.

4) Perhaps a security standard somewhere was devised that for some reason (good or bad) demanded this behavior; then it has been copied across the industry ever since.

The only justification I've ever seen was places gearing up for multi-factor authentication. The front login page, asking for your account, tells the backend how you're configured to login (token, password, biometric, whatever). Then the second page is variable depending on your authentication method.

> the complexity of [...] two fields

If that is true the world is doomed. Giant Meteor 2016

Many sites have started using "magic links" where you can choose to either receive an email with a link that will authenticate you or you can input the password.

Both Netflix and Slack does this.

I'm guessing to support company single sign-on. They lookup the domain of your email and then redirect to the correct SSO flow.