`login_url` lacks a mode hint

[panva]

There's only one login_url in the IdP configuration and it's shared for all modes.

But the modes can be configured differently in the IdP configuration, e.g.

{
  // ...
  "modes": {
    "button": {
      "supports_use_other_account": true,
    },
    "widget": {
      "supports_use_other_account": false,
    }
  }
}

Should there be an indication about the used mode at the login_url when it's invoked?

[npm1]

Hmm why though? User signing in to the IdP should not depend on the FedCM mode that the RP is requesting, should it?

[panva]

Hmm why though? User signing in to the IdP should not depend on the FedCM mode that the RP is requesting, should it?

For one it may be valuable to know the conversion of one over the other. Altho we do get the mode in the subsequent assertion call we can't tell the drop off in between the modes during login.

[tttzach]

Is this only for metrics purposes or do you have functional changes in mind? We do have plans for a metrics endpoint which at its current state doesn't include this mode information but we are open to hearing your use case.