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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. 
View Transition
A test case for View Transitions.
Setup
To reference a local build of React, first run npm run build at the root
of the React project. Then:
cd fixtures/view-transition
yarn
yarn start
The start command runs a webpack dev server and a server-side rendering server in development mode with hot reloading.
Note: whenever you make changes to React and rebuild it, you need to re-run yarn in this folder:
yarn
If you want to try the production mode instead run:
yarn start:prod
This will pre-build all static resources and then start a server-side rendering HTTP server that hosts the React app and service the static resources (without hot reloading).