[Fiber] Fix context propagation into Suspense fallbacks#36160
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acdlite merged 1 commit intofacebook:mainfrom Mar 27, 2026
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[Fiber] Fix context propagation into Suspense fallbacks#36160acdlite merged 1 commit intofacebook:mainfrom
acdlite merged 1 commit intofacebook:mainfrom
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When a context value changes above a Suspense boundary that is showing its fallback, context consumers inside the fallback do not re-render. They continue to display stale values. This happens because `propagateContextChanges`, upon encountering a suspended Suspense boundary, marks the boundary for retry but stops traversing into its children entirely (`nextFiber = null`). This skips both the hidden primary subtree (intentional — those fibers may not exist) and the visible fallback subtree (a bug — those fibers are committed and visible to the user). The fix skips the primary OffscreenComponent and continues traversal into the FallbackFragment, so fallback context consumers are found and marked for re-render. In practice this often goes unnoticed because it's uncommon to read context inside a Suspense fallback, and when some other update (like a prop change) flows into the fallback it sidesteps the propagation path entirely. React Compiler makes the bug more likely to surface since it memoizes more aggressively, reducing the chance of an incidental re-render masking the stale value.
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## Summary When a context value changes above a Suspense boundary that is showing its fallback, context consumers inside the fallback do not re-render — they display stale values. `propagateContextChanges`, upon encountering a suspended Suspense boundary, marks the boundary for retry but stops traversing into its children entirely (`nextFiber = null`). This skips both the hidden primary subtree (intentional — those fibers may not exist) and the visible fallback subtree (a bug — those fibers are committed and visible to the user). The fix skips the primary OffscreenComponent and continues traversal into the FallbackFragment, so fallback context consumers are found and marked for re-render. In practice this often goes unnoticed because it's uncommon to read context inside a Suspense fallback, and when some other update (like a prop change) flows into the fallback it sidesteps the propagation path entirely. React Compiler makes the bug more likely to surface since it memoizes more aggressively, reducing the chance of an incidental re-render masking the stale value. ## Test plan - Added regression test `'context change propagates to Suspense fallback (memo boundary)'` in `ReactContextPropagation-test.js` - Verified the test fails without the fix and passes with it - All existing context propagation, Suspense, memo, and hooks tests pass DiffTrain build for [9627b5a](9627b5a)
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## Summary When a context value changes above a Suspense boundary that is showing its fallback, context consumers inside the fallback do not re-render — they display stale values. `propagateContextChanges`, upon encountering a suspended Suspense boundary, marks the boundary for retry but stops traversing into its children entirely (`nextFiber = null`). This skips both the hidden primary subtree (intentional — those fibers may not exist) and the visible fallback subtree (a bug — those fibers are committed and visible to the user). The fix skips the primary OffscreenComponent and continues traversal into the FallbackFragment, so fallback context consumers are found and marked for re-render. In practice this often goes unnoticed because it's uncommon to read context inside a Suspense fallback, and when some other update (like a prop change) flows into the fallback it sidesteps the propagation path entirely. React Compiler makes the bug more likely to surface since it memoizes more aggressively, reducing the chance of an incidental re-render masking the stale value. ## Test plan - Added regression test `'context change propagates to Suspense fallback (memo boundary)'` in `ReactContextPropagation-test.js` - Verified the test fails without the fix and passes with it - All existing context propagation, Suspense, memo, and hooks tests pass DiffTrain build for [9627b5a](9627b5a)
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[diff facebook/react@3cb2c420...9627b5a1](facebook/react@3cb2c42...9627b5a) <details> <summary>React upstream changes</summary> - facebook/react#36160 - facebook/react#35701 - facebook/react#35627 - facebook/react#35621 - facebook/react#35623 </details> --------- Co-authored-by: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
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Summary
When a context value changes above a Suspense boundary that is showing its fallback, context consumers inside the fallback do not re-render — they display stale values.
propagateContextChanges, upon encountering a suspended Suspense boundary, marks the boundary for retry but stops traversing into its children entirely (nextFiber = null). This skips both the hidden primary subtree (intentional — those fibers may not exist) and the visible fallback subtree (a bug — those fibers are committed and visible to the user).The fix skips the primary OffscreenComponent and continues traversal into the FallbackFragment, so fallback context consumers are found and marked for re-render.
In practice this often goes unnoticed because it's uncommon to read context inside a Suspense fallback, and when some other update (like a prop change) flows into the fallback it sidesteps the propagation path entirely. React Compiler makes the bug more likely to surface since it memoizes more aggressively, reducing the chance of an incidental re-render masking the stale value.
Test plan
'context change propagates to Suspense fallback (memo boundary)'inReactContextPropagation-test.js