It’s indeed nice looking, but to tell what, here? Exiting the full screen composer is easy: The same icon you clicked to enter the mode is still there to click again to exit.
The discussion in this topic and in the other one ( There is no reply/save edit/save wiki button on the full screen composer - #5 by Mevo ) was about the lack of the repy/validation button in the full screen composer. Tell people they can use ESC + ENTER? Is this what you would want to show? (btw, this way of validating has all the exact same potential problems as the lacking button)