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This is like the onEnter/Exit/Share/Update events but for gestures. It allows manually controlling the animation using the passed timeline.

Follow up to #35337. During a gesture, we always cancel the original animation and create a new one that we control. That's the one we need to add to the set that needs to be cancelled. Otherwise future gestures hang. An unfortunate consequence is that any custom ones that you start e.g. with #35556 or through other means aren't automatically cleaned up (in fact there's not even a clean up callback yet). This can lead these to freeze the whole UI afterwards. It would be really good to get this fixed in browsers instead so we can revert #35337.

…ts (#35564) Stacked on #35556 and #35559. Given that we don't automatically clean up all view transition animations since #35337 and browsers are buggy, it's important that you clean up any `Animation` started manually from the events. However, there was no clean up function for when the View Transition is forced to stop. This also makes it harder to clean up custom timers etc too. This lets you return a clean up function from all the events on `<ViewTransition>`.

…g computed (#35565) `useInsertionEffect` is meant to be used to insert `<style>` tags that affect the layout. It allows precomputing a layout before it mounts. Since we're not normally firing any effects during the "apply gesture" phase where we create the clones, it's possible for the target snapshot to be missing styles. This makes it so that `useInsertionEffect` for a new tree are mounted before the snapshot is taken and then unmounted before the animation starts. Note that because we are mounting a clone of the DOM tree and the previous DOM tree remains mounted during the snapshot, we can't unmount any previous insertion effects. This can lead to conflicts but that is similar to what can happen with conflicts for two mounted Activity boundaries since insertion effects can remain mounted inside those. A revealed Activity will have already had their insertion effects fired while offscreen. However, one thing this doesn't yet do is handle the case where a `useInsertionEffect` is *updated* as part of a gesture being applied. This means it's still possible for it to miss some styles in that case. The interesting thing there is that since the old state and the new state will both be applicable to the global DOM in this phase, what should really happen is that we should mount the new updated state without unmounting the old state and then unmount the updated state. Meaning you can have the same hook in the mounted state twice at the same time.

Currently we always clone the root when a gesture transition happens. The was to add an optimization where if a Transition could be isolated to an absolutely positioned subtree then we could just clone that subtree or just do a plain insertion if it was simple an Enter. That way when switching between two absolutely positioned pages the shell wouldn't need to be cloned. In that case `detectMutationOrInsertClones` would return false. However, currently it always return true because we don't yet have that optimization. The idea was to warn when the root required cloning to ensure that you optimize it intentionally since it's easy to accidentally update more than necessary. However, since this is not yet actionable I'm removing this warning for now. Instead, I add a warning for particularly bad cases where you really shouldn't clone like iframe and video. They may not be very actionable without the optimization since you can't scope it down to a subtree without the optimization. So if they're above the gesture then they're always cloned atm. However, it might also be that it's unnecessary to keep them mounted if they could be removed or hidden with Activity.

Follow up to #35559. The clean up function of the custom timeline doesn't necessarily clean up the animation. Just the timeline's internal state. This affects Firefox which doesn't support ScrollTimeline so uses the polyfill's custom timeline.

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Commits on Jan 20, 2026

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