If you insist on using a single tab, you can do it as follows:
In particular, if you are not logged in, everything will look more-or-less normal until you try to enter a comment. Specifically: If no “Update Comments” button is visible, double-check that you are signed in. Look near the right of the top navigation bar. If your name appears, you are signed in; if it instead says “Sign In” then you need to sign in again. This is not a very clear indication. Bad UI design.
Again, this is terribly bad UI design.