css - Vertical scrollbar although big div is hidden -

I have 2 divisions: a login form and a register form. - The login form is quite small - the register form is longer than the browser's height.

But whenever the register div is hidden, I still have that vertical scroll bar ...

When a div is active (overflow-y: hidden) then What is the method to disable scrollbar only?

Edit: My code:

Please ensure that the vertical bar is being shown with the correct DIV element, possibly in the external device