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How do I make my pop up modal stick to the center of the screen?

I have made a pop up modal that looks and works great in preview on my read-only link. But when I publish the site, the modal pops up in the center of the site’s page, instead of at the center of my screen, so I have to scroll up or down to find it. My modal wrap is in a Full, Fixed position, which works just fine in preview. What do I need to change to make it work on my published site?