312407 2026-04-15 13:25:00 -0700 CSS linear() easing curve forces animations onto the main thread 2026-04-15 13:46:22 -0700 1 1 1 Unclassified WebKit Animations Safari 26 All iOS 26 RESOLVED DUPLICATE 312015 https://loading-text-jank-ncourygoogle.yoyo.codes P2 Normal --- 1 ncoury webkit-unassigned graouts oldest_to_newest 2200788 0 ncoury 2026-04-15 13:25:00 -0700 Reproduction at https://loading-text-jank-ncourygoogle.yoyo.codes Reproduces on iOS Safari and Desktop Safari 26. Description When working with CSS `linear()` easing curves (we are using them for spring physics), we found our animations are dropping a significant number of frames and janking severely, despite only utilizing transform and opacity to keep them on the compositor. We narrowed this down to the use of https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Values/easing-function/linear and if we use other easing curves the animation is properly run off the main thread. Digging for more details I noticed there's a history around this already: https://bugs.webkit.org/show_bug.cgi?id=240061 - Original implementation supported accelerated animations but contained bugs https://bugs.webkit.org/show_bug.cgi?id=260136 - Removed accelerated animation support for correctness https://bugs.webkit.org/show_bug.cgi?id=281429 - Marked as WAI in the short term until 250970 is resolved https://bugs.webkit.org/show_bug.cgi?id=250970 - Resolved, moving accelerated animations to another thread https://bugs.webkit.org/show_bug.cgi?id=307776 - Open bug identifying animations to run 60Hz+ Does this indicate there's a reasonable path to accelerating linear() curves again? Getting high polish spring physics is difficult on web without linear() + accelerated animations off the main thread at high refresh rates. 2200795 1 graouts 2026-04-15 13:34:07 -0700 This is true of the released Safari so far. However, once the "Time-based Threaded Animations" feature flag is enabled, this will won't be the case anymore. You can already try this in Safari Technology Preview where this flag is enabled by default. I can't however comment on when this will make it to a Safari release. 2200799 2 graouts 2026-04-15 13:38:13 -0700 Duping to bug 312015 where we enabled the relevant flag by default on all Cocoa ports. *** This bug has been marked as a duplicate of bug 312015 *** 2200802 3 graouts 2026-04-15 13:41:33 -0700 Note also that we shipped support for this for scroll-driven animations in macOS 26.4 and iOS 26.4. 2200806 4 ncoury 2026-04-15 13:46:22 -0700 Incredible! Just verified it works on STP. Look forward to it rolling out. And yes, we saw the 26.4 support and had actually been holding off on more aggressive use of scroll-driven animations until now. We're actively building out new animations on it and looking forward to the results. Thank you for all the help!