Okay so here is the demo site: http://hayati.yahyauddin.com/
To see the modal, hover over a product and click the zoom button to display the quick view for the product.
If you continue scrolling down, you will see that the Gray semi-transparent background stops. This problem occurs regardless of screen size!
Why does this happen and how can I fix this?
Note: This problem does not seem to occur in the foundation website: http://foundation.zurb.com/docs/components/reveal.html
Zurb guys changed its default position from fixed to absolute (i didn't understand why) then they changed their mind and went back to fixed again.
This was implemented in 5.5.1 https://github.com/zurb/foundation/commit/45f9b407ffeeca4b44c5a2afde4f0e58abf4bb78
updating will help :)