Fix: useDeferredValue should reuse previous value, not revert to initial value #24413
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During an urgent update, useDeferredValue should reuse the previous value. The regression test I added shows that it was reverting to the initial value instead. The cause of the bug was trivial: the update path doesn't update the hook's `memoizedState` field. Only the mount path. None of the existing tests happened to catch this because to trigger the bug, you have to do an urgent update that isn't the first update after initial render. In all of the existing tests that included an urgent update, it was the first update, so the "previous" value and the initial value happened to be the same thing.
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Summary: This sync includes the following changes: - **[bd4784c8f](facebook/react@bd4784c8f )**: Revert #24236 (Don't recreate the same fallback on the client if hydrating suspends) ([#24434](facebook/react#24434)) //<dan>// - **[6d3b6d0f4](facebook/react@6d3b6d0f4 )**: forwardRef et al shouldn't affect if props reused ([#24421](facebook/react#24421)) //<Andrew Clark>// - **[bd0813766](facebook/react@bd0813766 )**: Fix: useDeferredValue should reuse previous value ([#24413](facebook/react#24413)) //<Andrew Clark>// - **[9ae80d6a2](facebook/react@9ae80d6a2 )**: Suppress hydration warnings when a preceding sibling suspends ([#24404](facebook/react#24404)) //<Josh Story>// - **[0dc4e6663](facebook/react@0dc4e6663 )**: Land enableClientRenderFallbackOnHydrationMismatch ([#24410](facebook/react#24410)) //<Andrew Clark>// - **[354772952](facebook/react@354772952 )**: Land enableSelectiveHydration flag ([#24406](facebook/react#24406)) //<Andrew Clark>// - **[392808a1f](facebook/react@392808a1f )**: Land enableClientRenderFallbackOnTextMismatch flag ([#24405](facebook/react#24405)) //<Andrew Clark>// - **[1e748b452](facebook/react@1e748b452 )**: Land enableLazyElements flag ([#24407](facebook/react#24407)) //<Andrew Clark>// - **[4175f0593](facebook/react@4175f0593 )**: Temporarily feature flag numeric fallback for symbols ([#24401](facebook/react#24401)) //<Ricky>// - **[a6d53f346](facebook/react@a6d53f346 )**: Revert "Clean up Selective Hydration / Event Replay flag ([#24156](facebook/react#24156))" ([#24402](facebook/react#24402)) //<Ricky>// - **[ab9cdd34f](facebook/react@ab9cdd34f )**: Bugfix: In legacy mode, call suspended tree's unmount effects when it is deleted ([#24400](facebook/react#24400)) //<Andrew Clark>// - **[168da8d55](facebook/react@168da8d55 )**: Fix typo that happened during rebasing //<Andrew Clark>// - **[8bc527a4c](facebook/react@8bc527a4c )**: Bugfix: Fix race condition between interleaved and non-interleaved updates ([#24353](facebook/react#24353)) //<Andrew Clark>// - **[f7cf077cc](facebook/react@f7cf077cc )**: [Transition Tracing] Add Offscreen Queue ([#24341](facebook/react#24341)) //<Luna Ruan>// - **[4fc394bbe](facebook/react@4fc394bbe )**: Fix suspense fallback throttling ([#24253](facebook/react#24253)) //<sunderls>// - **[80170a068](facebook/react@80170a068 )**: Match bundle.name and match upper case entry points ([#24346](facebook/react#24346)) //<Sebastian Markbåge>// - **[fea6f8da6](facebook/react@fea6f8da6 )**: [Transition Tracing] Add transition to OffscreenState and pendingSuspenseBoundaries to RootState ([#24340](facebook/react#24340)) //<Luna Ruan>// - **[8e2f9b086](facebook/react@8e2f9b086 )**: move passive flag ([#24339](facebook/react#24339)) //<Luna Ruan>// - **[55a21ef7e](facebook/react@55a21ef7e )**: fix pushTransition for transition tracing ([#24338](facebook/react#24338)) //<Luna Ruan>// - **[069d23bb7](facebook/react@069d23bb7 )**: [eslint-plugin-exhaustive-deps] Fix exhaustive deps check for unstable vars ([#24343](facebook/react#24343)) //<Afzal Sayed>// - **[4997515b9](facebook/react@4997515b9 )**: Point useSubscription to useSyncExternalStore shim ([#24289](facebook/react#24289)) //<dan>// - **[01e2bff1d](facebook/react@01e2bff1d )**: Remove unnecessary check ([#24332](facebook/react#24332)) //<zhoulixiang>// - **[d9a0f9e20](facebook/react@d9a0f9e20 )**: Delete create-subscription folder ([#24288](facebook/react#24288)) //<dan>// - **[f993ffc51](facebook/react@f993ffc51 )**: Fix infinite update loop that happens when an unmemoized value is passed to useDeferredValue ([#24247](facebook/react#24247)) //<Andrew Clark>// - **[fa5800226](facebook/react@fa5800226 )**: [Fizz] Pipeable Stream Perf ([#24291](facebook/react#24291)) //<Josh Story>// - **[0568c0f8c](facebook/react@0568c0f8c )**: Replace zero with NoLanes for consistency in FiberLane ([#24327](facebook/react#24327)) //<Leo>// - **[e0160d50c](facebook/react@e0160d50c )**: add transition tracing transitions stack ([#24321](facebook/react#24321)) //<Luna Ruan>// - **[b0f13e5d3](facebook/react@b0f13e5d3 )**: add pendingPassiveTransitions ([#24320](facebook/react#24320)) //<Luna Ruan>// Changelog: [General][Changed] - React Native sync for revisions 60e63b9...bd4784c jest_e2e[run_all_tests] Reviewed By: kacieb Differential Revision: D35899012 fbshipit-source-id: 86a885e336fca9f0efa80cd2b8ca040f2cb53853
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During an urgent update, useDeferredValue should reuse the previous value. The regression test I added shows that it was reverting to the initial value instead.
The cause of the bug was trivial: the update path doesn't update the hook's
memoizedStatefield. Only the mount path.None of the existing tests happened to catch this because to trigger the bug, you have to do an urgent update that isn't the first update after initial render. In all of the existing tests that included an urgent update, it was the first update, so the "previous" value and the initial value happened to be the same thing.