Cache parent lookups during event dispatch (#56853)

[rubennorte]

Summary:

The W3C event-dispatch pipeline (gated on enableNativeEventTargetEventDispatching) walks the parent chain of the dispatched target on every event: once in EventTarget.getEventPath for the capture/bubble path, plus up to two additional walks in the responder system for touch/scroll/selection events. Each parentNode read is a JSI hop into C++ that re-walks the family chain in the current shadow-tree revision, so on event-heavy screens (e.g., a list firing many onLayout events during mount) the per-event walk cost adds up.

This change adds a per-instance parent cache that all event-dispatch consumers share. RN host trees are append-only and the shadow tree is stable during dispatch, so once a node is reachable from the dispatch path its parent is permanently stable from that pipeline's point of view. The cache stores the resolved parent in a symbol-keyed slot on the first lookup; subsequent lookups (within the same dispatch and across future dispatches on the same tree) collapse to a property load.

• Add getEventTargetParent(target) in EventTargetInternals (plus the cache slot and a sentinel for cached nulls).
• Route EventTarget.getEventPath through the new utility.
• Route the parentElement walks in ReactNativeResponder (getLowestCommonAncestor, negotiateResponder path build, and the skipSelf step) through the same utility, with an instanceof ReadOnlyElement filter so the responder's element-only invariant is preserved.
• Leave the parentNode getter on ReadOnlyNode untouched — user-visible reads still take the canonical JSI path and detached-node reads continue to return null.
• Add two stable-tree scenarios to EventDispatching-benchmark-itest.js (beforeAll mounts once, the benchmarked function dispatches per iteration) so the cache win is measurable and any regression in the rebuild-per-iter scenarios is also visible.

Benchmark results

Ran yarn fantom EventDispatching-benchmark --benchmarks from xplat/js/react-native-github/, comparing the cache disabled (utility short-circuited to the canonical getter) against the cache enabled. Numbers below are p50 latency; both runs use Hermes-bytecode optimized mode (the default for *-benchmark-itest.js).

New stable-tree scenarios with the new pipeline (enableNativeEventTargetEventDispatching ON):

Scenario (depth 50, stable tree)	Cache OFF	Cache ON	Improvement
dispatch event, bubbling, handlers on ancestors	0.271 ms	0.165 ms	39% faster (1.64×)
dispatch event, no handlers on ancestors	0.265 ms	0.161 ms	39% faster (1.65×)

Pre-existing rebuild-per-iter scenarios with the new pipeline (no measurable change — cache is empty on each iteration):

Scenario (flag ON)	Cache OFF	Cache ON	Δ
flat (1 handler)	0.042 ms	0.042 ms	—
nested 10 deep (bubbling)	0.105 ms	0.106 ms	—
nested 50 deep (bubbling)	0.378 ms	0.381 ms	—
nested 10 deep (no handlers)	0.103 ms	0.104 ms	—
stopPropagation, nested 10 deep	0.089 ms	0.091 ms	—
render + dispatch, flat	0.082 ms	0.083 ms	—

Legacy pipeline (enableNativeEventTargetEventDispatching OFF) was unchanged across both runs, confirming the cache change does not leak outside the new pipeline.

Changelog: [Internal]

Reviewed By: andrewdacenko

Differential Revision: D105337953

[meta-codesync]

@rubennorte has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105337953.

[react-native-bot]

This pull request was successfully merged by @rubennorte in e705e8d

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[meta-codesync]

This pull request has been merged in e705e8d.