Affected Versions:
Nightly 89.0a1

Preconditions
apz.overscroll.enabled true

Tested On:
MacOS 10.15/11

Steps to Reproduce:

1. Go to https://www.euronews.com
2. Scroll the top of the page in order to reproduce the overscroll effect

Expected Results:
The elastic effect is reproduced and causes no issues.

Actual Result:
The site has visual glitches whenever performing the elastic effect.

Note:
Not reproducible on Safari nor Chrome.
A recording of the issue can be found at : https://drive.google.com/file/d/1AYSjmlu3OiDragzBGv0Pe5tQGElj2Ghe/view?usp=sharing

I can reproduce sometimes but not others. When I can reproduce the body has class="dfp-wallpapered" and it has a fixed position background, which is the "visual glitch" that is described. I have not been able to get Safari or Chrome to get sent the version of the page with the class="dfp-wallpapered" body to see how they handle it.

Ah, adblock is the difference, as well the page layout changes based on the width of the browser. Our behaviour is different from Chrome: Chrome shows white, we reveal the fixed pos background in the overscroll gutter. Safari seems to have a built in adblock of some type that blocks the ad that I'm not sure how to disable to check.

I was able to get the ad from loading in Firefox after disabling all ad- and tracking blockers and agreeing to all cookies.
I also found how to disable tracking protection in Safari - see screenshot.

Safari as well as Chrome drag the background-attachment, which is set to fixed, down once they hit the top of the page, while it still "stays still" in Firefox.
Apparently Safari does not support background-attachment: fixed yet: https://developer.mozilla.org/en-US/docs/Web/CSS/background-attachment#browser_compatibility
Removing this property in Firefox makes it behave like the other two.
Leaves us with Chrome ... maybe they internally change the background-attachment: fixed value to something else?

This is the CSS

@media screen and (min-width: 64em) {
  .dfp-wallpapered.has-breakingNews-visible {
    background-position: center 3.125rem;
  }
}

@media (min-width: 1280px) {
  .dfp-wallpapered {
    background: #ffffff url(https://tpc.googlesyndication.com/simgad/7246920193261549974?) 50% 0 no-repeat;
    background-attachment: fixed;
    background-position: center 0;
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
  }
}

Closing this as resolved:worksforme. We can no longer reproduce this on the latest Firefox Nightly 90.0a1 nor Beta 89.0b8.

I'm assuming that's because the site changed and the underlying issue still exists.