[Fiber] Don't disappear Layout Effects when hydrating <Activity mode="hidden">

[eps1lon]

We use fiber.memoizedState === null for to determine if the Offscreen Fiber is hidden. That state was initialized for hidden Offscreen everywhere except when hydrating hidden Offscreen when we bail out.

This lead to attempting to disappear Layout Effects. This works just fine now since there are no destroy functions registered just yet. So this is mostly for correctness. Perf concerns are negligible since the hidden Offscreen in Activity wouldn't have any child Fibers.

I only noticed this when trying to use offscreenSubtreeWasHidden in #34983. When hydrating <Activity mode="hidden">, we entered recursivelyTraverseDisappearLayoutEffects in

react/packages/react-reconciler/src/ReactFiberCommitWork.js

Lines 2543 to 2559 in 7b22afd

	if (isHidden) {
	// Only trigger disappear layout effects if:
	// - This is an update, not first mount.
	// - This Offscreen was not hidden before.
	// - Ancestor Offscreen was not hidden in previous commit or in this commit
	if (
	isUpdate &&
	!wasHidden &&
	!offscreenSubtreeIsHidden &&
	!offscreenSubtreeWasHidden
	) {
	if (
	disableLegacyMode ||
	(finishedWork.mode & ConcurrentMode) !== NoMode
	) {
	// Disappear the layout effects of all the children
	recursivelyTraverseDisappearLayoutEffects(finishedWork);

[react-sizebot]

Comparing: 63e95c2...7b22afd

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	+0.01%	614.26 kB	614.33 kB	=	108.57 kB	108.57 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	+0.01%	680.19 kB	680.26 kB	=	119.51 kB	119.52 kB
facebook-www/ReactDOM-prod.classic.js	+0.01%	700.61 kB	700.68 kB	=	123.09 kB	123.10 kB
facebook-www/ReactDOM-prod.modern.js	+0.01%	690.93 kB	691.00 kB	=	121.48 kB	121.48 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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