[Bugfix] Don't double invoke effects for list reorders

[everettbu]

Mirror of facebook/react#35952
Original author: rickhanlonii

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Fixes react/react#32561

[greptile-apps]

Greptile Summary

Fixes a bug where StrictMode incorrectly double-invoked effects on keyed list reorders. The root cause was that placeChild in ReactChildFiber.js set the PlacementDEV flag for moved children (reorders), not just new insertions. Since ReactFiberWorkLoop.js uses PlacementDEV to decide which fibers get their effects double-invoked, this caused spurious unmount/remount cycles on reorder. The fix removes PlacementDEV from the move code path while preserving it for genuine insertions.

• Removed PlacementDEV from the move branch in placeChild, keeping only the Placement flag needed for DOM reordering
• Added three edge-case tests covering pure reorders, reorders mixed with new insertions, and reorders mixed with removals

Confidence Score: 5/5

• This PR is safe to merge — it is a minimal, targeted bugfix with comprehensive test coverage.
• The change is a single-line removal of one flag from one code path, with clear semantic justification: moves are not insertions and should not trigger StrictMode double-invoke. The fix is well-supported by three new tests covering key edge cases. The PlacementDEV flag remains correctly applied for genuine insertions and single-child placements. No regressions are expected.
• No files require special attention.

Important Files Changed

Filename	Overview
packages/react-reconciler/src/ReactChildFiber.js	Removes PlacementDEV flag from the move branch of placeChild, so moved keyed children no longer trigger StrictMode double-invoke of effects. Insertions (line 536) and single child placements (line 545) correctly retain PlacementDEV.
packages/react-reconciler/src/tests/StrictEffectsMode-test.js	Adds three well-structured edge-case tests: pure reorder (no effect re-fire), reorder with a new insertion (only new item double-invoked), and reorder with a removal (only removed item's cleanup fires).

_{Last reviewed commit: e05c062}

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