Hi,
I think its worth to make modal window a bit higher to fit entire form (including CONFIRM button) at once.

Otherwise
- on desktop its confusing because it is not so clear that the button does not fit window size and the content needs be scrolled up.
- on mobile you end up with srollable window inside scrollable window, a bit messy

thanks

Modal can have only ~90% height of screen...

I didn't know that.

Maybe as for updated version you would think about to redesign the form slightly ?
If you will recover those 20-30px of height, confirmation button would fit in whole without the scroll bar.


EDIT:
As a quick fix:

In the file oc-content/themes/starter/css/style.css
changing .ur-row value on line 98
from: padding: 10px 15px;
to: padding: 7px 15px;

will make enough room for the button to appear in whole without the scroll bar.
But this change might also change some other elements layout on main page. I didn't tested.