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[Fiber] Double invoke Effects in Strict Mode during Hydration - #35961
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[diff react/react@fef12a01...da9325b5](react/react@fef12a0...da9325b) <details> <summary>React upstream changes</summary> - react/react#35962 - react/react#35961 - react/react#36277 - react/react#36297 - react/react#36292 - react/react#36291 - react/react#36196 - react/react#36246 - react/react#36287 - react/react#36285 - react/react#36274 - react/react#36266 - react/react#36253 </details>
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…-invoke StrictMode's dev-only double invoke of effects (doubleInvokeEffectsOnFiber) runs disappearLayoutEffects + reappearLayoutEffects on a fiber that never actually leaves the tree. For HostSingletons (<html>/<head>/<body>) those paths call commitHostSingletonRelease/commitHostSingletonAcquisition, which destructively remove every attribute from the live DOM node and re-apply only React's props. Since react#35961 marks the hydrated HostRoot with PlacementDEV, the root subtree is now double-invoked on initial hydration, so the root <html> singleton gets released+re-acquired right after hydration. This wipes any attribute an inline script set before hydration (e.g. the no-flash-of-incorrect-theme pattern that seeds a class on <html> from localStorage). It is dev + StrictMode only; production is unaffected. The double invoke is meant to exercise effects and refs, not to re-mount host singletons (acquisition is a placement/DOM concern). Guard the singleton release/acquire with a new isRunningDoubleInvokeEffects flag set around the simulation, so the live singleton's attributes are preserved. Ref attach/detach still double-invokes via the HostComponent fallthrough.
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…#92945) [diff react/react@fef12a01...da9325b5](react/react@fef12a0...da9325b) <details> <summary>React upstream changes</summary> - react/react#35962 - react/react#35961 - react/react#36277 - react/react#36297 - react/react#36292 - react/react#36291 - react/react#36196 - react/react#36246 - react/react#36287 - react/react#36285 - react/react#36274 - react/react#36266 - react/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?