[Fiber] Double invoke Effects in Strict Mode during Hydration#35961
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eps1lon merged 2 commits intofacebook:mainfrom Apr 17, 2026
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[Fiber] Double invoke Effects in Strict Mode during Hydration#35961eps1lon merged 2 commits intofacebook:mainfrom
eps1lon merged 2 commits intofacebook:mainfrom
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[diff facebook/react@fef12a01...da9325b5](facebook/react@fef12a0...da9325b) <details> <summary>React upstream changes</summary> - facebook/react#35962 - facebook/react#35961 - facebook/react#36277 - facebook/react#36297 - facebook/react#36292 - facebook/react#36291 - facebook/react#36196 - facebook/react#36246 - facebook/react#36287 - facebook/react#36285 - facebook/react#36274 - facebook/react#36266 - facebook/react#36253 </details>
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Summary
Also reported in vercel/next.js#66210
Not double invoking during Hydration was on oversight. Double invoking during Hydration ensures parity between
hydrateRootandcreateRoot.Double invoking an Effect is tracked with
PlacementDEV. This was always used together withPlacement. However, during hydration we don't track side-effects i.e. set noPlacementflag causing no double invocation of Effects in Strict Mode.Now we set
PlacementDEVwhen we set theHydrationflag to double invoke Effects in Strict Mode when we hydrated which is conceptually like a Placement during client-render.How did you test this change?