Fix crash from withheld exiting layout animations in experimental proxy

[tomekzaw]

Summary

Closes #9822.

On iOS release builds with ENABLE_SHARED_ELEMENT_TRANSITIONS: true (the default on main), the Suspense + Layout Animation Crash example crashes within ~1–2 seconds with EXC_BAD_ACCESS inside LayoutAnimationsProxy_Experimental::updateLightTree. This is the experimental-proxy counterpart of #9820/#9821: when React re-creates a tag whose exiting removal the proxy is still withholding (a re-suspending Suspense boundary deletes a subtree and later re-creates it with the same fiber tags), the proxy's bookkeeping is contradicted and corrupts both the light tree and the mounting layer's view registry.

Unlike the legacy proxy — whose nodeForTag_ tracks every node of a withheld subtree, so the #9821 reconcile could catch any re-created tag — the experimental proxy only kept ANIMATING roots in lightNodes_. Two whole classes of withheld views were invisible to any reconcile:

• WAITING interior nodes (e.g. FlatList cell containers kept alive under an exiting item) — tracked in neither lightNodes_ nor deadNodes,
• settled DEAD nodes — already moved to deadNodes and erased from lightNodes_, but still mounted until the end-of-transaction cleanup, which runs after an incoming Create would have re-registered the tag.

A Create for such a tag passed straight through against a still-registered old view. Diagnosing with a mutation-stream replay showed the resulting "double Create" (an iOS RCTComponentViewRegistry overwrite) as the single divergence source; the wrong view then got recycled by the old node's deferred Delete, and subsequent transactions faulted on null light nodes / stale indices.

The fix mirrors #9821, adapted to the experimental proxy's bookkeeping:

• reconcileContradictedRemovals runs at the start of pullTransaction: an incoming Create/Insert targeting a withheld tag — found in lightNodes_ (any non-live state) or in deadNodes — flushes that node's withheld Remove/Delete immediately, before the tag is re-registered, mirroring the deadNodes cleanup in handleRemovals.
• WAITING subtree members are now registered in lightNodes_ (both the kept-subview branch of startAnimationsRecursively and the kept-root branch of handleRemovals), so the reconcile can find them — matching the legacy proxy's per-node bookkeeping.
• endAnimationsRecursively and maybeDropAncestors erase the deleted node's lightNodes_ entry (guarded so a re-registered live node is never touched). This also fixes a pre-existing stale-entry leak for animating descendants force-ended together with an exiting ancestor.
• endLayoutAnimation replaces an unchecked lightNodes_[tag] lookup with a guarded find — the old code would default-insert a null node and dereference it in release, where the assert is compiled out.

Test plan

Uses the Suspense + Layout Animation Crash example added in #9821, with ENABLE_SHARED_ELEMENT_TRANSITIONS left at its default (true):

1. build fabric-example for iOS in Release,
2. open the example and tap Start stress.

Before this change the app crashes within ~1–2 seconds; three consecutive runs on main produced the exact stack reported in #9822 (SIGSEGV in updateLightTree ← pullTransaction ← RCTMountingManager performTransaction). After it, the same stress ran through hundreds of mode switches (several minutes) with no crash, and a replay of ~525k logged mutations from an instrumented run showed no registry overwrites and no index divergence between the emitted mutations and the mounted hierarchy.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 6491f3e7-98b8-47a0-8be5-0c93495f8c71

📥 Commits

Reviewing files that changed from the base of the PR and between fcd31e1 and 066e79d.

📒 Files selected for processing (1)

• packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Fixed layout animations when views are rapidly removed and re-created, including cases where pending exits are contradicted by new inserts.
  • Prevented stale exit/removal logic from deleting views that were already reinserted.
  • Hardened exit animation finalization to avoid incorrect node lookups that could leave inconsistent state.
  • Improved reliability and cleanup for complex nested view transitions during fast layout changes.

Walkthrough

The experimental layout animation proxy reconciles re-created tags with withheld removals during mounting transactions and keeps lightNodes_ mappings synchronized across removal, animation, and ancestor cleanup paths.

Changes

Layout removal reconciliation

Layer / File(s)	Summary
Contradicted removal reconciliation packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.*	pullTransaction reconciles Create and Insert mutations against withheld or dead nodes, removes contradicted nodes from their parents, and ends subtree animations before ancestor cleanup.
Light-node lifecycle cleanup packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp	Removal and animation paths register waiting nodes, erase matching stale mappings, and safely handle missing entries when ending layout animations.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related issues

• #9402 — Covers related mutation reconciliation and stale-node cleanup in the layout animation proxy.

Suggested reviewers: bartlomiejbloniarz

Sequence Diagram(s)

sequenceDiagram
  participant ReactMounting
  participant LayoutAnimationsProxy_Experimental
  participant lightNodes_
  participant deadNodes
  participant LightNodeParent
  ReactMounting->>LayoutAnimationsProxy_Experimental: pullTransaction(mutations)
  LayoutAnimationsProxy_Experimental->>lightNodes_: reconcile Create/Insert tags
  LayoutAnimationsProxy_Experimental->>deadNodes: check DEAD nodes
  LayoutAnimationsProxy_Experimental->>LightNodeParent: remove contradicted node
  LayoutAnimationsProxy_Experimental->>LayoutAnimationsProxy_Experimental: end subtree animations and drop ancestors
  LayoutAnimationsProxy_Experimental-->>ReactMounting: filtered mutations

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🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: fixing a crash in the experimental proxy caused by withheld exiting layout animations.
Description check	✅ Passed	The description matches the reported crash scenario and the implemented fix, so it is clearly related to the PR.
Linked Issues check	✅ Passed	The changes implement the #9822 fix by reconciling contradicted removals and hardening light-node bookkeeping in the experimental proxy.
Out of Scope Changes check	✅ Passed	The diff stays focused on the experimental layout-animation crash fix and related bookkeeping changes.

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• Commit unit tests in branch @tomekzaw/fix-withheld-exiting-experimental-proxy

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🧹 Nitpick comments (1)

packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp (1)

123-172: 📐 Maintainability & Code Quality | 🔵 Trivial | 🏗️ Heavy lift

Consider regression test coverage for this crash path.

This is a targeted fix for a hard-to-hit, timing-dependent native crash (Suspense re-suspension + exiting layout animations + shared element transitions). No test exercising reconcileContradictedRemovals/the recreate-while-withheld scenario is included in the reviewed files. If there's an existing native/mounting-layer test harness for this proxy, a regression test would materially reduce risk of silent reintroduction.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In
`@packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp`
around lines 123 - 172, Add a regression test using the existing native or
mounting-layer harness for LayoutAnimationsProxy_Experimental and exercise
reconcileContradictedRemovals when a tag is recreated while its removal is
withheld, including the exiting/shared-transition timing path. Assert the stale
removal is flushed and the recreated view remains correctly registered without
crashing.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In
`@packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp`:
- Around line 123-172: Add a regression test using the existing native or
mounting-layer harness for LayoutAnimationsProxy_Experimental and exercise
reconcileContradictedRemovals when a tag is recreated while its removal is
withheld, including the exiting/shared-transition timing path. Assert the stale
removal is flushed and the recreated view remains correctly registered without
crashing.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 31024595-4185-4564-8145-e498872f0c94

📥 Commits

Reviewing files that changed from the base of the PR and between cf5f300 and fcd31e1.

📒 Files selected for processing (2)

• packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.cpp
• packages/react-native-reanimated/Common/cpp/reanimated/LayoutAnimations/LayoutAnimationsProxy_Experimental.h