The feature is the combination of the page level configuration and the dynamic sign in callback. This simplify managing configuration for a page, makes managing sign-in state easier, and generally provide a much better feel for the user.
Page Level Config
As you can see this allows you to define the callback as well, so you really don’t need to pass any options to a sign-in button:
Dynamic Sign-In Callback
This is the one that really improves the experience for me though. One of the general problems developers faced was how to trigger an immediate mode check. If you display the sign-in button, then it can be there for a while before the check has completed, which looks a bit odd to a returning user. The normal method was to hide the sign-in button off screen until the check had been completed. With this change, the callback will fire on initial page load.
On top of that, it also tracks the sign-in state of the user. Try visiting the example page in an incognito window, sign-in, and then in another tab sign out of Google. You'll see an error immediate_failed_user_logged_out. The reverse would happen if you signed-in using the same browser instance, and had previously granted access. Very cool!
This means we can drive behaviour directly from the callback, which actually makes the code simpler. Here we are just checking the state and displaying the appropriate buttons. We don’t need to worry too much about whether the user is signed-out, not logged in to Google, automatically signed-in or manually signed in. Just as an example, we’ll tweak the message depending on whether they consented or were seamlessly signed-in.
The important thing to note in the callback is that we’re not triggering this by creating the button - we only call gapi.render when we have a confirmed signed-out state, so our flow is much more cleanly encapsulated.