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Prevent my mobile Nav from scrolling

Dear forum.
My clients website has a dropdown menu, which has some dropdown links in it.

When i open the menu it all works fine, but when i scroll down this happens:

As you can see, when i scroll, i can see the page underneath the nav menu.
A user sent me a script so i can disable scrolling, but that wont help, because i have to be able to scroll, because of the dropdown links, that will lengthen the whole nav menu.

Can you please help me with a good solution?
Is there a way to make the nav menu cover the whole screen and still be able to be scrolled through?

Thanks and Blessings!

Here is my site Read-Only: LINK
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