Add ScrollTimeline Polyfill for Swipe Recognizer using a new CustomTimeline protocol #33501
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This is also how you'd implement other gestures that can't be modeled as scroll. Such as rotate and zoom gestures. |
sebmarkbage
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| @@ -5,6 +5,8 @@ import React, { | |||
| unstable_startGestureTransition as startGestureTransition, | |||
| } from 'react'; | |||
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| import ScrollTimelinePolyfill from 'animation-timelines/scroll-timeline'; | |||
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I published this as a third party package instead since it's generally useful and I can put more complex ones there.
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This basically lets a custom implementation drive the Animation we start on pseudo-elements.
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Stacked on #33501. This disables the use of ScrollTimeline when detected in Safari in the recommended SwipeRecognizer approach. I'm instead using a polyfill using touch events on iOS. Safari seems set to [release ScrollTimeline soon](https://webkit.org/blog/16993/news-from-wwdc25-web-technology-coming-this-fall-in-safari-26-beta/). Unfortunately it's not really what you'd expect. First of all, [it's not running in sync with the scroll](https://bugs.webkit.org/show_bug.cgi?id=288402) which is kind of its main point. Instead, it is running at 60fps and out of sync with the scroll just like JS. In fact, it is worse than JS because with JS you can at least spawn CSS animations that run at 120fps. So our polyfill can respond to touches at 60fps while gesturing and then run at 120fps upon release. That's better than with ScrollTimeline. Second, [there's a bug which interrupts scrolling if you start a ViewTransition](https://bugs.webkit.org/show_bug.cgi?id=288795) when the element is being removed as part of that. The element can still respond to touches so in a polyfill this isn't an issue. But it essentially makes it useless to use ScrollTimeline with swipe-away gestures. So we're better off in every scenario by not using it. The UA detection is a bit unfortunate. Not sure if there's something more specific but we also had to do a UA detection for Chrome for View Transitions. Those are the only two we have in all of React. 
This was referenced Jul 3, 2025
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The React API is just that we now accept this protocol as an alternative to a native
AnimationTimelineto be passed tostartGestureTransition. This is specifically the DOM version.Instead, of passing this to the
Animationthat we start to control the View Transition keyframes, we instead inverse the control and pass theAnimationto this one. It lets any custom implementation drive the updates. It can do so by updating the time every frame or letting it run a time based animation (such as momentum scroll).In this case I added a basic polyfill for
ScrollTimelinein the example but we'll need a better one.