fix: startTransition in popstate should not show Suspense fallback

[everettbu]

Mirror of facebook/react#35975
Original author: fresh3nough

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Summary

When startTransition is called inside a popstate event 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 getNextLanesToFlushSync adds an artificial SyncLane to the render lanes as a priority marker. This causes includesOnlyTransitions() to return false, which breaks two critical checks:

1. renderDidSuspendDelayIfPossible() does not set workInProgressRootIsPrerendering = true, so renderRootSync does not yield when the shell suspends
2. finishConcurrentRender() commits the fallback for RootSuspendedWithDelay instead of suspending indefinitely

Fix

Add a flag (workInProgressRootIsEagerPopstateTransition) that tracks when the current render was initiated as a popstate eager transition. This flag is used alongside the existing includesOnlyTransitions() check in:

• renderDidSuspendDelayIfPossible: enables prerendering mode so renderRootSync yields when the shell suspends
• finishConcurrentRender: prevents committing the fallback, falling through to RootSuspendedAtTheShell behavior

This 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 fallback that verifies:

• The transition is attempted synchronously during the popstate microtask
• The Suspense fallback is NOT shown (previous UI remains visible)
• After the promise resolves, the new UI is rendered

All existing popstate, Suspense, and transition tests continue to pass.

Fixes #35966

[greptile-apps]

Greptile Summary

Fixes a bug where startTransition called inside a popstate event handler would incorrectly show the Suspense fallback instead of keeping the previous UI visible when the component suspends.

• The root cause is that getNextLanesToFlushSync adds an artificial SyncLane to popstate eager transition render lanes, causing includesOnlyTransitions() to return false. This broke two critical code paths: renderDidSuspendDelayIfPossible (which should enable prerendering mode) and finishConcurrentRender (which should suspend indefinitely instead of committing the fallback).
• The fix introduces a workInProgressRootIsEagerPopstateTransition flag that is captured at the start of each render via prepareFreshStack and used alongside the existing includesOnlyTransitions() check in both affected code paths. The flag has a clean lifecycle — it's set from currentSyncTransitionIsPopstate in the root scheduler (which is scoped to the microtask flush) and re-evaluated on every fresh render.
• Includes a thorough test case that verifies the previous UI remains visible during suspension, the fallback is pre-warmed but not committed, and the new UI renders correctly after the promise resolves.

Confidence Score: 5/5

• This PR is safe to merge — it's a well-scoped bug fix with correct flag lifecycle management and a comprehensive test.
• The changes are minimal and precisely targeted. The two new flags (currentSyncTransitionIsPopstate in the scheduler and workInProgressRootIsEagerPopstateTransition in the work loop) have clean lifecycles: the scheduler flag is set only during the popstate microtask flush and unconditionally reset afterward; the work loop flag is re-evaluated on every fresh render via prepareFreshStack, ensuring it doesn't leak across renders. The fix is additive — it only widens the conditions under which React treats a suspended render as a transition (keeping previous UI), and does not alter any existing code paths for non-popstate transitions. The test thoroughly validates the expected behavior.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react-reconciler/src/ReactFiberRootScheduler.js	Adds currentSyncTransitionIsPopstate flag that is set when a popstate eager transition is being flushed, and reset unconditionally after the flush. Clean, well-scoped change.
packages/react-reconciler/src/ReactFiberWorkLoop.js	Adds workInProgressRootIsEagerPopstateTransition flag captured in prepareFreshStack, used in renderDidSuspendDelayIfPossible and finishConcurrentRender to treat popstate eager transitions like normal transitions when they suspend. Flag lifecycle is correct — reset on each new render.
packages/react-dom/src/tests/ReactDOMFiberAsync-test.js	Adds comprehensive test verifying that startTransition inside a popstate event handler does not show Suspense fallback when the component suspends, matching the behavior of non-popstate transitions.

_{Last reviewed commit: 7ad96ed}