Pop-up not overlaying elements on the front page

I try to place a Photonic gallery in the sidebar. However, when I click an image of an album to display the photos, the pop-up panel does not overlay all the page elements (e.g. the original album featured images of Photonic are on the top as well as some other parts of the website).

The issue is on the following website: https://www.skmbrussels.be/domovska-stranka-test/.

Many thanks in advance for your guidance on how to fix this.

Vladimir