Hi,
I have noticed long time ago this and thought maybe later it will get fixed but as for now it seems not.
You need to be logged in. If you are in new checkout and wait until session expires. Pressing on any button with ajax it will load home screen inside that div with following error messages. "You session has expired", "The cart is empty".
Or if you try to proceed to next step it will give invalid token on the left top screen with blank page.
I have attached image where i used to enter coupon after session has expired.