[pull] main from facebook:main #71
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This adds navigation support to the View Transition fixture using both `history.pushState/popstate` and the Navigation API models. Because `popstate` does scroll restoration synchronously at the end of the event, but `startViewTransition` cannot start synchronously, it would observe the "old" state as after applying scroll restoration. This leads to weird artifacts. So we intentionally do not support View Transitions in `popstate`. If it suspends anyway for some other reason, then scroll restoration is broken anyway and then it is supported. We don't have to do anything here because this is already how things worked because the sync `popstate` special case already included the sync lane which opts it out of View Transitions. For the Navigation API, scroll restoration can be blocked. The best way to do this is to resolve the Navigation API promise after React has applied its mutation. We can detect if there's currently any pending navigation and wait to resolve the `startViewTransition` until it finishes and any scroll restoration has been applied. https://github.com/user-attachments/assets/f53b3282-6315-4513-b3d6-b8981d66964e There is a subtle thing here. If we read the viewport metrics before scroll restoration has been applied, then we might assume something is or isn't going to be within the viewport incorrectly. This is evident on the "Slide In from Left" example. When we're going forward to that page we shift the scroll position such that it's going to appear in the viewport. If we did this before applying scroll restoration, it would not animate because it wasn't in the viewport then. Therefore, we need to run the after mutation phase after scroll restoration. A consequence of this is that you have to resolve Navigation in `useInsertionEffect` as otherwise it leads to a deadlock (which eventually gets broken by `startViewTransition`'s timeout of 10 seconds). Another consequence is that now `useLayoutEffect` observes the restored state. However, I think what we'll likely do is move the layout phase to before the after mutation phase which also ensures that auto-scrolling inside `useLayoutEffect` are considered in the viewport measurements as well.
…on (#32029) This allows mutations and scrolling in the layout phase to be counted towards the mutation. This would maybe not be the case for gestures but it is useful for fire-and-forget. This also avoids the issue that if you resolve navigation in useLayoutEffect that it ends up dead locked. It also means that useLayoutEffect does not observe the scroll restoration and in fact, the scroll restoration would win over any manual scrolling in layout effects. For better or worse, this is more in line with how things worked before and how it works in popstate. So it's less of a breaking change. This does mean that we can't unify the after mutation phase with the layout phase though. To do this we need split out flushSpawnedWork from the flushLayoutEffect call. Spawned work from setState inside the layout phase is done outside and not counted towards the transition. They're sync updates and so are not eligible for their own View Transitions. It's also tricky to support this since it's unclear what things like exits in that update would mean. This work will still be able to mutate the live DOM but it's just not eligible to trigger new transitions or adjust the target of those. One difference between popstate is that this spawned work is after scroll restoration. So any scrolling spawned from a second pass would now win over scroll restoration. Another consequence of this change is that you can't safely animate pseudo elements in useLayoutEffect. We'll introduce a better event for that anyway.
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