"Sign Up" button is unseen in Opera when all fields are filled incorrectly

1. Go to Forum, press Sign up button
2. In the opened Sign up screen press "Sign up" button in order to get error messages
3. Type any combination in "Confirm password" field in order to get one more error message

As a result, Sign Up window has become too long to fit the screen, "Sign Up" button is not seen and it is not possible to scroll to this button (please, see video attached)
Expected result: Sign Up window should fit the screen, all fields and buttons should be seen, as it happens in Chrome (please, see screenshot attached)