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[flags] Add enableParallelTransitions - #35392
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rickhanlonii
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Dec 22, 2025
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@diffray-bot thanks, you've been banned. |
sebmarkbage
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Jan 12, 2026
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I ran into an interesting issue in the View Transition fixture once we unentangle. If there's another Transition that can commit while we have started a Navigation, then we'll wait for that Navigation to finish before we can commit it. However, it might be blocked by a different Navigation which is going to unblock it with its We have that wait so that scroll restoration and other browser state can resolve properly before the animation finishes but there's no need to wait for it unless this is the Transition that's going to be unblocking it. We'd need to somehow know which Navigation is connected to which Transition. |
This was referenced Jan 12, 2026
sebmarkbage added a commit
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Jan 16, 2026
…35486) Stacked on #35485. Before this PR, the `startGestureTransition` API would itself never commit its state. After the gesture releases it stops the animation in the next commit which just leaves the DOM tree in the original state. If there's an actual state change from the Action then that's committed as the new DOM tree. To avoid animating from the original state to the new state again, this is DOM without an animation. However, this means that you can't have the actual action committing be in a slightly different state and animate between the final gesture state and into the new action. Instead, we now actually keep the render tree around and commit it in the end. Basically we assume that if the Timeline was closer to the end then visually you're already there and we can commit into that state. Most of the time this will be at the actual end state when you release but if you have something else cancelling the gesture (e.g. `touchcancel`) it can still commit this state even though your gesture recognizer might not consider this an Action. I think this is ok and keeps it simple. When the gesture lane commits, it'll leave a Transition behind as work from the revert lanes on the Optimistic updates. This means that if you don't do anything in the Action this will cause another commit right after which reverts. This revert can animate the snap back. There's a few fixes needed in follow up PRs: - Fixed in #35487. ~To support unentangled Transitions we need to explicitly entangle the revert lane with the Action to avoid committing a revert followed by a forward instead of committing the forward entangled with the revert. This just works now since everything is entangled but won't work with #35392.~ - Fixed in #35510. ~This currently rerenders the gesture lane once before committing if it was already completed but blocked. We should be able to commit the already completed tree as is.~
sebmarkbage added a commit
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…35487) Stacked on #35486. When a Gesture commits, it leaves behind work on a Transition lane (`revertLane`). This entangles that lane with whatever lane we're using in the event that cancels the Gesture. This ensures that the revert and the result of any resulting Action commits as one batch. Typically the Action would apply a new state that is similar or the same as the revert of the Gesture. This makes it resilient to unbatching in #35392.
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…35486) Stacked on #35485. Before this PR, the `startGestureTransition` API would itself never commit its state. After the gesture releases it stops the animation in the next commit which just leaves the DOM tree in the original state. If there's an actual state change from the Action then that's committed as the new DOM tree. To avoid animating from the original state to the new state again, this is DOM without an animation. However, this means that you can't have the actual action committing be in a slightly different state and animate between the final gesture state and into the new action. Instead, we now actually keep the render tree around and commit it in the end. Basically we assume that if the Timeline was closer to the end then visually you're already there and we can commit into that state. Most of the time this will be at the actual end state when you release but if you have something else cancelling the gesture (e.g. `touchcancel`) it can still commit this state even though your gesture recognizer might not consider this an Action. I think this is ok and keeps it simple. When the gesture lane commits, it'll leave a Transition behind as work from the revert lanes on the Optimistic updates. This means that if you don't do anything in the Action this will cause another commit right after which reverts. This revert can animate the snap back. There's a few fixes needed in follow up PRs: - Fixed in #35487. ~To support unentangled Transitions we need to explicitly entangle the revert lane with the Action to avoid committing a revert followed by a forward instead of committing the forward entangled with the revert. This just works now since everything is entangled but won't work with #35392.~ - Fixed in #35510. ~This currently rerenders the gesture lane once before committing if it was already completed but blocked. We should be able to commit the already completed tree as is.~ DiffTrain build for [4028aaa](4028aaa)
github-actionsBot
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Jan 16, 2026
…35486) Stacked on #35485. Before this PR, the `startGestureTransition` API would itself never commit its state. After the gesture releases it stops the animation in the next commit which just leaves the DOM tree in the original state. If there's an actual state change from the Action then that's committed as the new DOM tree. To avoid animating from the original state to the new state again, this is DOM without an animation. However, this means that you can't have the actual action committing be in a slightly different state and animate between the final gesture state and into the new action. Instead, we now actually keep the render tree around and commit it in the end. Basically we assume that if the Timeline was closer to the end then visually you're already there and we can commit into that state. Most of the time this will be at the actual end state when you release but if you have something else cancelling the gesture (e.g. `touchcancel`) it can still commit this state even though your gesture recognizer might not consider this an Action. I think this is ok and keeps it simple. When the gesture lane commits, it'll leave a Transition behind as work from the revert lanes on the Optimistic updates. This means that if you don't do anything in the Action this will cause another commit right after which reverts. This revert can animate the snap back. There's a few fixes needed in follow up PRs: - Fixed in #35487. ~To support unentangled Transitions we need to explicitly entangle the revert lane with the Action to avoid committing a revert followed by a forward instead of committing the forward entangled with the revert. This just works now since everything is entangled but won't work with #35392.~ - Fixed in #35510. ~This currently rerenders the gesture lane once before committing if it was already completed but blocked. We should be able to commit the already completed tree as is.~ DiffTrain build for [4028aaa](4028aaa)
github-actionsBot
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that referenced
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Jan 16, 2026
…35487) Stacked on #35486. When a Gesture commits, it leaves behind work on a Transition lane (`revertLane`). This entangles that lane with whatever lane we're using in the event that cancels the Gesture. This ensures that the revert and the result of any resulting Action commits as one batch. Typically the Action would apply a new state that is similar or the same as the revert of the Gesture. This makes it resilient to unbatching in #35392. DiffTrain build for [35a81ce](35a81ce)
| }, | ||
| ); | ||
| it( |
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@acdlite here's the tests with suspended siblings. It looks like in either case Final is pre-warmed, but with parallel transitions we don't pre-warm the inital value.
enableParallelTransitions experiment to entangle less oftenenableParallelTransitions experiment to entangle less oftenenableParallelTransitions| case TransitionLane10: | ||
| if (enableParallelTransitions) { | ||
| return getHighestPriorityLane(lanes); | ||
| } |
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@acdlite I'm only doing this for the transition lanes, should we also do it for the deferred lanes?
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Overview
Adds a feature flag
enableParallelTransitionsto experiment with engantling transitions less often.Motivation
Currently we over-entangle transition lanes.
It's a common misunderstanding that React entangles all transitions, always. We actually will complete transitions independently in many cases. For example, this codepen from @gabbev shows transitions completing independently.
However, in many cases we entangle when we don't need to, instead of letting the independent transitons complete independently. We still want to entangle for updates that happen on the same queue.
Example
As an example of what this flag would change, consider two independent counter components:
Before
The behavior today is to entange them, meaning they always commit together:
Screen.Recording.2026-01-22.at.11.12.34.AM.mov
After
In this experiment, they will complete independently (if they don't depend on each other):
Screen.Recording.2026-01-22.at.11.15.01.AM.mov
Early Research
This change is in early research, and is not in the experimental channel. We're going to experiment with this at Meta to understand how much of a breaking change, and how beneficial it is before commiting to shipping it in experimental and beyond.