[Android][Fabric] Create mutation missing from an unflatten reparenting batch under remount churn — production evidence for the #47960 class (surface-killing)

Framing: this appears to be a concrete, reproducible, production-scale instance of #47960 ("Transaction merging can cause CREATE mutations to be effectively dropped", closed as not_planned) — filed with fresh evidence because the impact is a dead surface at ~2% of Android sessions, the graceful-handling attempts (#56654, #56389, #56376) are unmerged, and the differentiator-side fixes that DID land (LIS differ, parentTag fixes, present in 0.85.3) do not prevent it.

Summary

When a flattened (layout-only) View must be unflattened in a commit that races with remount churn of the same subtree, the emitted IntBufferBatchMountItem can contain the reparenting sequence for the materialized wrapper without its Create mutation. SurfaceMountingManager.addViewAt then throws RetryableMountingLayerException: Unable to find viewState for tag, and since batch items are not retried (only view commands are), MountItemDispatcher.dispatchMountItems rethrows and the host tears the instance down — on a production app this is a dead white screen (or a hard crash under expo-updates error recovery).

Evidence

Captured on device (RN 0.85.3, bridgeless, Galaxy S23 Ultra, Android 16). The failing batch, printed by MountItemDispatcher:

REMOVE [2180]->[14] @3     // existing child removed from its parent...
INSERT [2184]->[14] @3     // ...a NEW view takes its slot — but 2184 has no viewState:
INSERT [2180]->[2184] @0   //    no Create/preallocation anywhere for it
UPDATE PROPS [2180]
UPDATE STATE [2180]
UPDATE LAYOUT [2184]->[14]: x:0 y:0 w:1080 h:2316
UPDATE LAYOUT [2180]->[2184]: x:0 y:0 w:1080 h:2316
UPDATE EVENTEMITTER [2184]
UPDATE EVENTEMITTER [2180]

printSurfaceState at crash time confirms tag 2184 is absent from the registry (526 views listed; 2184 not among them), i.e. the Create was never emitted or executed — this is not an ordering problem within the batch.

The shape is the classic unflatten sequence: child keeps its tag and reparents into the newly materialized wrapper. The wrapper here is a library-owned layout-only view (StyleSheet.absoluteFill + pointerEvents="box-none") around a native sheet component, whose sibling/outer wrapper toggles pointerEvents between 'none' and 'auto' — the prop change that makes it non-flattenable.

Causal confirmation: forcing the outermost wrapper real with collapsable={false} moved the identical missing-Create crash exactly one level down to the next flattenable wrapper in the ancestor chain (REMOVE [3010]->[3016]; INSERT [3014]->[3016]; INSERT [3010]->[3014] — 3016 now materialized and registered, 3014 missing). Pinning every wrapper in the chain eliminates the crash entirely (0/40 reproductions vs ~1/3 before).

Trigger conditions (from ~150 production events + device repro)

• Android, New Architecture (Fabric), RN 0.85.3 (production telemetry also on 0.85.x; the sub-signature first appeared for us after adopting native-stack screens, and scaled ~20x when a high-frequency bottom-sheet moved onto this wrapper shape).
• A flattenable wrapper whose subtree is rapidly remounted (portal entry torn down and recreated within a frame or two — e.g. reopening a sheet before the previous instance finished closing) while a prop change forces the wrapper to unflatten.
• Reproduces within ~3 attempts on device with that churn; the same UI used slowly does not reproduce.

Where the Create is likely lost

The differentiator emitting Insert-without-Create seems unlikely given the unflatten fixes already present in 0.85.3 (LIS differ #56094, parentTag fixes #50884/#56542 — all included, crash persists). The shape instead matches transaction merging (#44188 / #47960): the unflatten commit's Create lands in one transaction, the churn (portal entry unmount/remount within a frame or two, main thread busy with the sheet teardown) produces adjacent transactions, and the merge drops the Create while keeping the Insert/reparent. We have not instrumented MountingCoordinator::pullTransaction to prove the merge directly, but the trigger profile (only under rapid remount churn; never when the same UI is used slowly) is consistent with it.

Repro

Minimal standalone repro not yet extracted; the real-world trigger is @swmansion/react-native-bottom-sheet's portal host (issue filed there with the same evidence: software-mansion-labs/react-native-bottom-sheet#78, one-line fix in software-mansion-labs/react-native-bottom-sheet#79), where each portal entry is <View style={absoluteFill} pointerEvents="box-none"> around a native sheet, remounted under user churn. On a Galaxy S23 Ultra this reproduced within ~3 open/act/reopen cycles. Happy to work toward a minimal repro if the mutation traces above are not sufficient to locate the path.

Expected

A batch must never contain an Insert for a tag that no executed transaction has Created — regardless of transaction merging or concurrent remount churn. Secondarily: a missing viewState in the batch-mount path destroys the entire React instance in bridgeless mode (white screen, app unusable); the graceful-handling direction of #56654 would at least bound the blast radius.

Workaround

collapsable={false} on every flattenable ancestor wrapper of the affected subtree (prevents the flatten→unflatten transition entirely).

Start at MountingCoordinator::pullTransaction and trace transaction merging into MountItemDispatcher.dispatchMountItems and SurfaceMountingManager.addViewAt, using the captured batch sequence as the failing case. Reproduce the flatten/unflatten remount churn on Android; done means no batch inserts an uncreated tag and a missing viewState does not tear down the instance. A minimal standalone repro is not yet available.