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Before someone says this is a duplicate of Why does it says "welcome back -user- click here to 'refresh page'" -- hold on a sec. I have one question to tack on to this.
The careers page: http://careers.stackoverflow.com/ logs in automatically, instantly.
Is there something different about the careers system which makes implementing this easier?
Yes, there is.
The careers login is similar to the login for https://chat.stackoverflow.com – since both of these are subdomains of stackoverflow.com, they have access to the Stack Overflow cookie (if you're logged in to SO). Hence it is possible for careers (and chat) to identify you immediately when the request comes in.
The auto-login to SE sites has to do a little more, because once you log in to superuser.com, we want to auto-login you when you visit askubuntu.com – and this obviously does not work with cookies. So this auto-login requires a little detour; in particular, it's not possible to do it within a single request.
