Login Flow: redirect back to article if user is already logged in

Given the user is logged in via the publisher domain. On mobile safari third-party cookies are disabled by default. No access to cookies mean that when the user views an article via the CDN, the publisher backend is not able to determine whether the user is logged in.

When the user clicks the login button, the login flow opens on the publisher domain. There the login flow is able to determine a logged in user and should redirect the user back to the article (after saving the readerid user mapping).

Our sample doesn't support this use case yet and shows the login dialog.

//cc @dvoytenko @ashwinlimaye @andreban