Self-expanding background under Zoom Image Lightbox - Why?

Can someone explain to me why the background under the displayed image (the image triggered by On Click - Zoom Image in Lightbox) slightly enlarges, and after closing the image, the background returns to its normal size?
The background images below the thumbnails are Wide Box - Fixed in windows - Stretch Fill.
This is a website made for my client.

I’m thinking @GRAFKOM the best way to find out more is to explain the situation to @duncan at

I have tried to see if it happens to me on macOS Big Sur Safari and I can’t replicate your description.

Something on the page isn’t built/formatted properly.

As you scroll down the page and the pencil starts to move to the right I can slide the entire page off to the left and see white “background”. That shouldn’t happen. I think this is the cause for the background slightly enlarging because the entire element isn’t “fixed” in it’s boundaries.