Failing test for double invoking Effects during hydration

[eps1lon]

Working in render but broken in hydrateRoot.

[react-sizebot]

Comparing: 5f06c3d...314aff8

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	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.18 kB	530.18 kB	=	93.39 kB	93.39 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	655.81 kB	655.81 kB	=	115.30 kB	115.30 kB
facebook-www/ReactDOM-prod.classic.js	=	675.58 kB	675.58 kB	=	118.54 kB	118.54 kB
facebook-www/ReactDOM-prod.modern.js	=	666.00 kB	666.00 kB	=	116.86 kB	116.87 kB
react-native/shims/ReactNativeViewConfigRegistry.js	=	3.69 kB	3.55 kB	=	1.32 kB	1.23 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
react-native/shims/ReactNativeViewConfigRegistry.js	=	3.69 kB	3.55 kB	=	1.32 kB	1.23 kB

Generated by 🚫 dangerJS against 314aff8

[eps1lon]

Uses mountChildFibers which has side-effect tracking disabled. Side-effect tracking sets the PlacementDEV flag. Since PlacementDEV is never set here, we never double invoke:

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

Lines 3691 to 3701 in 94eed63

	if (fiber.flags & PlacementDEV) {
	setCurrentDebugFiberInDEV(fiber);
	if (isInStrictMode) {
	doubleInvokeEffectsOnFiber(
	root,
	fiber,
	(fiber.mode & NoStrictPassiveEffectsMode) === NoMode,
	);
	}
	resetCurrentDebugFiberInDEV();
	} else {

But then it's not clear yet why we need this flag at all. Shouldn't we double invoke regardless of whether something was placed?

[eps1lon]

Landed in #35961