Need help with horizontal positioning


I would love for someone to figure this one out with me.

So I made a horizontal scroll site concept. Which Is kinda like a museum. you get past all these frames and when you click one a modal pops up and gives some information. But I cant get the modal to keep in the center of the screen when you scroll or detect where you are on the site.

Exampe; when you scroll to the far right and click a frame to open up the modal it pops up to the far left.

How would I fix this issue?

Hope to hear some feedback

Here is my site Read-Only: LINK