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[wip] experiment to entangle less often #35392

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ce470de
Add feature flag
rickhanlonii Dec 19, 2025
289a320
Don't entangle all transition lanes
rickhanlonii Dec 19, 2025
d59601f
Mark entangled lanes as suspended
rickhanlonii Jan 6, 2026
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4 changes: 4 additions & 0 deletions packages/react-reconciler/src/ReactFiberLane.js
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@ import {
disableLegacyMode,
enableDefaultTransitionIndicator,
enableGestureTransition,
enableParallelTransitions,
} from 'shared/ReactFeatureFlags';
import {isDevToolsPresent} from './ReactFiberDevToolsHook';
import {clz32} from './clz32';
Expand Down Expand Up @@ -208,6 +209,9 @@ function getHighestPriorityLanes(lanes: Lanes | Lane): Lanes {
case TransitionLane8:
case TransitionLane9:
case TransitionLane10:
if (enableParallelTransitions) {
return getHighestPriorityLane(lanes);
}
return lanes & TransitionUpdateLanes;
case TransitionLane11:
case TransitionLane12:
Expand Down
6 changes: 6 additions & 0 deletions packages/react-reconciler/src/ReactFiberWorkLoop.js
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,7 @@ import {
enableViewTransition,
enableGestureTransition,
enableDefaultTransitionIndicator,
enableParallelTransitions,
} from 'shared/ReactFeatureFlags';
import {resetOwnerStackLimit} from 'shared/ReactOwnerStackReset';
import ReactSharedInternals from 'shared/ReactSharedInternals';
Expand Down Expand Up @@ -1719,6 +1720,11 @@ function markRootSuspended(
spawnedLane: Lane,
didAttemptEntireTree: boolean,
) {
if (enableParallelTransitions) {
// When suspending, we should always mark the entangled lanes as suspended.
suspendedLanes = getEntangledLanes(root, suspendedLanes);
}

// When suspending, we should always exclude lanes that were pinged or (more
// rarely, since we try to avoid it) updated during the render phase.
suspendedLanes = removeLanes(suspendedLanes, workInProgressRootPingedLanes);
Expand Down
18 changes: 12 additions & 6 deletions packages/react-reconciler/src/__tests__/ReactDeferredValue-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -421,8 +421,10 @@ describe('ReactDeferredValue', () => {
// also suspends.
'Suspend! [Final]',
// pre-warming
'Suspend! [Loading...]',
'Suspend! [Final]',
...(gate('enableParallelTransitions')
? // TODO: this loses pre-warming, maybe that's fine?
[]
: ['Suspend! [Loading...]', 'Suspend! [Final]']),
]);
expect(root).toMatchRenderedOutput(null);

Expand Down Expand Up @@ -463,8 +465,10 @@ describe('ReactDeferredValue', () => {
// also suspends.
'Suspend! [Final]',
// pre-warming
'Suspend! [Loading...]',
'Suspend! [Final]',
...(gate('enableParallelTransitions')
? // TODO: this loses pre-warming, maybe that's fine?
[]
: ['Suspend! [Loading...]', 'Suspend! [Final]']),
]);
expect(root).toMatchRenderedOutput(null);

Expand Down Expand Up @@ -540,8 +544,10 @@ describe('ReactDeferredValue', () => {
// also suspends.
'Suspend! [Final]',
// pre-warming
'Suspend! [Loading...]',
'Suspend! [Final]',
...(gate('enableParallelTransitions')
? // TODO: this loses pre-warming, maybe that's fine?
[]
: ['Suspend! [Loading...]', 'Suspend! [Final]']),
]);
expect(root).toMatchRenderedOutput(null);

Expand Down
313 changes: 313 additions & 0 deletions packages/react-reconciler/src/__tests__/ReactTransition-test.js
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Expand Up @@ -209,6 +209,319 @@ describe('ReactTransition', () => {
expect(root).toMatchRenderedOutput('Async');
});

// @gate enableLegacyCache
it('when multiple transitions update different queues, they entangle', async () => {
let setA;
let startTransitionA;
let setB;
let startTransitionB;
function A() {
const [a, _setA] = useState(0);
const [isPending, _startTransitionA] = useTransition();
setA = _setA;
startTransitionA = _startTransitionA;

return (
<span>
{isPending && (
<span>
<Text text="Pending A..." />
</span>
)}
<AsyncText text={`A: ${a}`} />
</span>
);
}

function B() {
const [b, _setB] = useState(0);
const [isPending, _startTransitionB] = useTransition();
setB = _setB;
startTransitionB = _startTransitionB;

return (
<span>
{isPending && (
<span>
<Text text="Pending B..." />
</span>
)}
<AsyncText text={`B: ${b}`} />
</span>
);
}
function App() {
return (
<>
<Suspense fallback={<span>Loading A</span>}>
<A />
</Suspense>
<Suspense fallback={<span>Loading B</span>}>
<B />
</Suspense>
</>
);
}

// Initial render
const root = ReactNoop.createRoot();
await act(() => {
root.render(<App />);
});
assertLog([
'Suspend! [A: 0]',
'Suspend! [B: 0]',
'Suspend! [A: 0]',
'Suspend! [B: 0]',
]);
expect(root).toMatchRenderedOutput(
<>
<span>Loading A</span>
<span>Loading B</span>
</>,
);

// Resolve
await act(() => {
resolveText('A: 0');
resolveText('B: 0');
});
assertLog(['A: 0', 'B: 0']);
expect(root).toMatchRenderedOutput(
<>
<span>A: 0</span>
<span>B: 0</span>
</>,
);

// Start transitioning A
await act(() => {
startTransitionA(() => {
setA(1);
});
});
assertLog(['Pending A...', 'A: 0', 'Suspend! [A: 1]']);
expect(root).toMatchRenderedOutput(
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>B: 0</span>
</>,
);

// Start transitioning B
await act(() => {
startTransitionB(() => {
setB(1);
});
});
assertLog(['Pending B...', 'B: 0', 'Suspend! [A: 1]', 'Suspend! [B: 1]']);
expect(root).toMatchRenderedOutput(
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>
<span>Pending B...</span>B: 0
</span>
</>,
);

// Resolve B
await act(() => {
resolveText('B: 1');
});
assertLog(
gate('enableParallelTransitions')
? ['B: 1', 'Suspend! [A: 1]']
: ['Suspend! [A: 1]', 'B: 1'],
);
expect(root).toMatchRenderedOutput(
gate('enableParallelTransitions') ? (
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>B: 1</span>
</>
) : (
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>
<span>Pending B...</span>B: 0
</span>
</>
),
);

// Resolve A
await act(() => {
resolveText('A: 1');
});
assertLog(gate('enableParallelTransitions') ? ['A: 1'] : ['A: 1', 'B: 1']);
expect(root).toMatchRenderedOutput(
<>
<span>A: 1</span>
<span>B: 1</span>
</>,
);
});

// @gate enableLegacyCache
it('when multiple transitions update different queues, but suspend the same boundary, they do entangle', async () => {
let setA;
let startTransitionA;
let setB;
let startTransitionB;
function A() {
const [a, _setA] = useState(0);
const [isPending, _startTransitionA] = useTransition();
setA = _setA;
startTransitionA = _startTransitionA;

return (
<span>
{isPending && (
<span>
<Text text="Pending A..." />
</span>
)}
<AsyncText text={`A: ${a}`} />
</span>
);
}

function B() {
const [b, _setB] = useState(0);
const [isPending, _startTransitionB] = useTransition();
setB = _setB;
startTransitionB = _startTransitionB;

return (
<span>
{isPending && (
<span>
<Text text="Pending B..." />
</span>
)}
<AsyncText text={`B: ${b}`} />
</span>
);
}
function App() {
return (
<Suspense fallback={<span>Loading...</span>}>
<A />
<B />
</Suspense>
);
}

// Initial render
const root = ReactNoop.createRoot();
await act(() => {
root.render(<App />);
});
assertLog([
'Suspend! [A: 0]',
// pre-warming
'Suspend! [A: 0]',
'Suspend! [B: 0]',
]);
expect(root).toMatchRenderedOutput(<span>Loading...</span>);

// Resolve
await act(() => {
resolveText('A: 0');
resolveText('B: 0');
});
assertLog(['A: 0', 'B: 0']);
expect(root).toMatchRenderedOutput(
<>
<span>A: 0</span>
<span>B: 0</span>
</>,
);

// Start transitioning A
await act(() => {
startTransitionA(() => {
setA(1);
});
});
assertLog(['Pending A...', 'A: 0', 'Suspend! [A: 1]']);
expect(root).toMatchRenderedOutput(
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>B: 0</span>
</>,
);

// Start transitioning B
await act(() => {
startTransitionB(() => {
setB(1);
});
});
assertLog(['Pending B...', 'B: 0', 'Suspend! [A: 1]', 'Suspend! [B: 1]']);
expect(root).toMatchRenderedOutput(
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>
<span>Pending B...</span>B: 0
</span>
</>,
);

// Resolve B
await act(() => {
resolveText('B: 1');
});
assertLog(
gate('enableParallelTransitions')
? ['B: 1', 'Suspend! [A: 1]']
: ['Suspend! [A: 1]', 'B: 1'],
);
expect(root).toMatchRenderedOutput(
gate('enableParallelTransitions') ? (
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>B: 1</span>
</>
) : (
<>
<span>
<span>Pending A...</span>A: 0
</span>
<span>
<span>Pending B...</span>B: 0
</span>
</>
),
);

// Resolve A
await act(() => {
resolveText('A: 1');
});
assertLog(gate('enableParallelTransitions') ? ['A: 1'] : ['A: 1', 'B: 1']);
expect(root).toMatchRenderedOutput(
<>
<span>A: 1</span>
<span>B: 1</span>
</>,
);
});

// @gate enableLegacyCache
it(
'when multiple transitions update the same queue, only the most recent ' +
Expand Down
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