Shifting background on mobile device

Hi, I use Nivo Lightbox on my website, and I noticed something: when I open on mobile device any image in post (at least in Google Chrome), and scroll to the top of the page (with image still opened in lighbox), and then fastly swipe down, then the darkened background shifts, and a space with no background appears at the bottom for some time, before it shifts back: https://ibb.co/wFWjkK3n

It doesn’t happen on my friends mobile device (he checked in Safari), so I guess it depends either on mobile browser, or screen width/height.

I had the same problem with my popup’s background, but I was able to fix it by adding extra padding for darkened background, if I remember correctly.

Please, fix it.