Any way to make a static background stay while you scroll?

Apologies if this is a bit hard to understand.

Some sites have a background that fills the screen... until you scroll. There's one on my work's intranet, so obviously I can't use that as an example. But I see the same thing on my Lemmy instance, db0 (not mine mine, the one I use). They made this thing, I think it's meant to be a black hole, heavily blurred, but when I scroll, the image scrolls away.

Is there some way to lock the background in place and let the content just scroll past it? Maybe with a setting, or more likely with an extension?