1720501 - CSS animations stop working for viewports >= 2048px

Status: NEW

(Core :: Layout, defect)

Description

[[e-mail]]

CSS animations appear to be broken when the viewport width is >= 2048px on Nightly. I'm using Nightly on OSX (see version string below).

STR:

• Using nightly (I'm using 92.0a1 (2021-07-14) (64-bit))
• Go to https://extensionworkshop.com/
• Set your viewport to 2047px wide or lower using responsive mode.
• Scroll down and the top nav bar will transition out of view.
• Increase your viewport size to 2048px or higher
• Try scrolling down, and the top nav bar disappears rather than animating.

What happens:

The animation is broken when the viewport width is >= 2048px.

What should happen:

The nav bar should smoothly animate out of view when the user scrolls down.

Other info

Running mozRegression pointed to https://bugzilla.mozilla.org/show_bug.cgi?id=1656418 and re-testing having set layout.animation.prerender.partial to false the animation works when the viewport width is over 2048px.