fix: startTransition in popstate should not show Suspense fallback#35975
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fresh3nough wants to merge 1 commit intofacebook:mainfrom
Open
fix: startTransition in popstate should not show Suspense fallback#35975fresh3nough wants to merge 1 commit intofacebook:mainfrom
fresh3nough wants to merge 1 commit intofacebook:mainfrom
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When startTransition is called inside a popstate event handler, the popstate eager transition path adds an artificial SyncLane to the render lanes. This caused includesOnlyTransitions() to return false, which prevented the suspension from being handled like a normal transition - showing the Suspense fallback instead of keeping the previous UI visible. Add a flag (workInProgressRootIsEagerPopstateTransition) that tracks when the current render was initiated as a popstate eager transition. Use this flag in renderDidSuspendDelayIfPossible and finishConcurrentRender to treat the render like a transition despite the artificial SyncLane, falling back to async behavior when the component suspends. Fixes: facebook#35966 Signed-off-by: Ubuntu <[email protected]>
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Comparing: 4610359...7ad96ed Critical size changesIncludes critical production bundles, as well as any change greater than 2%:
Significant size changesIncludes any change greater than 0.2%: (No significant changes) |
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Summary
When
startTransitionis called inside apopstateevent handler and the component suspends inside a<Suspense>boundary, React incorrectly shows the fallback instead of keeping the previous UI visible.Root Cause
The popstate eager transition path in
getNextLanesToFlushSyncadds an artificialSyncLaneto the render lanes as a priority marker. This causesincludesOnlyTransitions()to returnfalse, which breaks two critical checks:renderDidSuspendDelayIfPossible()does not setworkInProgressRootIsPrerendering = true, sorenderRootSyncdoes not yield when the shell suspendsfinishConcurrentRender()commits the fallback forRootSuspendedWithDelayinstead of suspending indefinitelyFix
Add a flag (
workInProgressRootIsEagerPopstateTransition) that tracks when the current render was initiated as a popstate eager transition. This flag is used alongside the existingincludesOnlyTransitions()check in:renderDidSuspendDelayIfPossible: enables prerendering mode sorenderRootSyncyields when the shell suspendsfinishConcurrentRender: prevents committing the fallback, falling through toRootSuspendedAtTheShellbehaviorThis ensures the popstate transition falls back to normal async transition behavior when it cannot complete synchronously.
Test Plan
Added a new test case
popstate transition with Suspense boundary should not show fallbackthat verifies:All existing popstate, Suspense, and transition tests continue to pass.
Fixes #35966