I've come across this a couple of times and it seems to be some 'clever' code trying to position the box part way down the list of variations but fails to take into account the size of the screen and the fact it locks out the scrolling whilst being a modal button.
The way I solved it was to use the browser's zoom controls to shrink the page until the button came into view again.
Now, in theory, it should be possible to use the TAB and Space-bar to press on the button even if you can't see it but I was never comfortable making changes by stabbing at controls I couldn't see.