Question: 

When products are added to cart, the popup appears below the dark screen.



Answer:

In this case, it is assumed that's because of conflict between M2 Add Multiple Product to Cart and M2 Ajax Add to Cart. To solve this issue, could you please edit the file below on your site?

This is the path of file:
/public_html/app/code/Bss/AjaxCart/view/frontend/web/css/jquery.fancybox.css

In line 38, please change the value from  “z-index: 8030” to “z-index: 9999”.


Please note that, it's just the solution for your reference.


For more information about product, please refer Add Multiple Product to Cart for Magento 2 and Ajax Add to Cart for Magento 2 page.

If you need any further assistance, please feel free to contact us.

Please visit our site for more useful Magento 2 Extensions: https://bsscommerce.com/magento-2-extensions.html