Pop Up Pro Mobile Scrolling

On the homepage there is a button labelled "free gift" which actions pop pro on click.

The weirdest thing is that when you scroll - horizontal or vertical - the page behind moves.

Perhaps not many people will scroll a form that they just have to add details too but for those that do I think it looks a bit buggy/unfinished.

Is there a way to prevent the page behind moving on scroll?

Thanks!