[Fiber] Double invoke Effects in StrictMode after Fast Refresh

[everettbu]

Mirror of facebook/react#35962
Original author: eps1lon

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Summary

Fixes react/react#29915

Ensures Effects are also double invoked when a Component newly mounts due to Fast Refresh. Looks to have been an oversight in the remountFiber path.

How did you test this change?

• Added test (see first commit for previous behavior)

[greptile-apps]

Greptile Summary

This PR fixes an oversight where the remountFiber path (used during Fast Refresh) did not set the PlacementDEV flag on newly created fibers. Without this flag, StrictMode's double-invocation of effects was skipped for components remounted via Fast Refresh, contrary to the behavior for all other newly mounted components.

• Adds PlacementDEV flag in remountFiber to match the existing pattern in ReactChildFiber.js
• Adds two tests covering double invocation of effects during Fast Refresh in StrictMode (existing effect and newly added effect)
• Change is entirely within a __DEV__ guard, so there is zero production impact

Confidence Score: 5/5

• This PR is safe to merge — it's a minimal, well-scoped DEV-only bug fix with test coverage.
• The code change is a single-line addition of a flag that already exists in all analogous code paths. The remountFiber function is entirely gated by __DEV__, so there is no production risk. Two comprehensive tests validate the expected behavior. The fix aligns with the established pattern in ReactChildFiber.js.
• No files require special attention.

Important Files Changed

Filename	Overview
packages/react-reconciler/src/ReactFiberBeginWork.js	Adds PlacementDEV flag alongside Placement in remountFiber, matching the pattern already used in ReactChildFiber.js. This ensures newly mounted fibers via Fast Refresh are recognized for StrictMode double-invocation of effects. The function is gated by __DEV__, so there is no production impact.
packages/react-refresh/src/tests/ReactFresh-test.js	Adds two new test cases verifying that effects are double-invoked in StrictMode after a Fast Refresh remount: one where the effect existed before the refresh, and one where the effect is newly added during the refresh. Tests follow existing patterns and use proper act/patch helpers.

_{Last reviewed commit: 71c0f2a}