[Hooks] Keep hydrated content on post-hydration useSyncExternalStore patch-up (tests)#36592
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[Hooks] Keep hydrated content on post-hydration useSyncExternalStore patch-up (tests)#36592devjiwonchoi wants to merge 1 commit into
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…e sync Land the tests for enableHoldHydratedContentOnStoreSync before the flag and implementation. When the post-hydration useSyncExternalStore patch-up (the sync re-render that reconciles the client snapshot against the server snapshot we hydrated with) suspends an already revealed Suspense boundary, the hydrated content should be kept instead of flashing the fallback. Coverage: - core keep-content, gated on/off - StrictMode double-invoked passive effects do not clear the hydration marker - marker is cleared so a later steady-state update still flashes - no hold when client and server snapshots match at hydration - hold spans the whole reconcile window, not just the first update - reconciler scoping guard: a steady-state sync update still flashes The gated assertions fall through to today's flash behavior until the flag and implementation land, so this commit passes on its own. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
Draft, test-first. This PR adds the tests for an experimental feature behind
enableHoldHydratedContentOnStoreSync(__EXPERIMENTAL__); the implementation lands in a follow-up. The tests pin the intended behavior.Behavior: when the post-hydration
useSyncExternalStorepatch-up (the sync re-render that reconciles the client snapshot against the server snapshot we hydrated with) re-suspends an already-revealed Suspense boundary, keep the hydrated content instead of committing the fallback, treating that sync store update as delay-able like a transition. Scoped to the hydration reconcile window: steady-state sync store updates still flash the fallback. The hold covers the whole window (multiple in-window updates, not just the first), and is root-wide within the patch-up commit (atomic commit, like a transition).Trade-off: while held, the boundary keeps content derived from the server snapshot even though the store has advanced to the client snapshot, so other non-suspending readers of the same store can show the newer value until the held boundary resolves.
Tests
packages/react-dom/src/__tests__/ReactDOMFizzShellHydration-test.js:packages/react-reconciler/src/__tests__/useSyncExternalStoreSuspenseFlicker-test.js:The gated assertions fall through to today's flash behavior until the flag and implementation land, so the suite is green on its own.
How did you test this?
yarn testandyarn test-stablefor both files: 16/16 in each channel.🤖 Generated with Claude Code