fix: don't double-invoke effects for moved children in StrictMode

[emmaeng700]

Summary

Fixes #32561 — in React 19, StrictMode was incorrectly re-running effects (even with [] dependencies) when a keyed child is reordered within an array. This does not happen in React 18 or in production builds.

Root Cause

placeChild() in ReactChildFiber.js sets PlacementDEV on both newly inserted fibers and moved fibers (those with an existing alternate whose oldIndex < lastPlacedIndex). The StrictMode double-invoke logic in doubleInvokeEffectsInDEVIfNecessary treats any fiber with PlacementDEV as a new mount and re-runs its effects.

A moved fiber is not a new mount — it already has state, refs, and a lifecycle. Re-running its effects in dev mode is incorrect and diverges from production behavior.

Fix

Only set PlacementDEV for actual insertions (current === null). Moved fibers still receive the Placement flag so the host node is repositioned in the DOM, but PlacementDEV is omitted so StrictMode does not treat them as new mounts.

// Before
if (oldIndex < lastPlacedIndex) {
  // This is a move.
  newFiber.flags |= Placement | PlacementDEV; // <- wrong
}

// After
if (oldIndex < lastPlacedIndex) {
  // This is a move — DOM node moves but component is not newly mounting.
  newFiber.flags |= Placement; // PlacementDEV intentionally omitted
}

Test

Added a test in StrictEffectsMode-test.js that renders [A, B, C] in StrictMode, reorders to [C, A, B], and asserts that no effect callbacks fire (no mount/unmount of useEffect or useLayoutEffect with [] deps).

Checklist

• Reproduces the bug with a new test
• Fix is minimal and targeted — one line change in placeChild()
• No behavior change in production (the PlacementDEV path is dev-only)
• No behavior change for actual new insertions

[react-sizebot]

Comparing: 98ce535...9a86561

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	=	611.76 kB	611.79 kB	+0.01%	108.10 kB	108.11 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	=	677.69 kB	677.72 kB	+0.01%	119.07 kB	119.08 kB
facebook-www/ReactDOM-prod.classic.js	+0.08%	697.89 kB	698.44 kB	+0.11%	122.63 kB	122.76 kB
facebook-www/ReactDOM-prod.modern.js	+0.08%	688.21 kB	688.76 kB	+0.11%	121.01 kB	121.15 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 9a86561