WorkerHandler blocks the main thread under event bursts #20547

WorkerHandler.postMessage in @sentry-internal/replay attaches a fresh 'message' listener to the compression worker for every request, and removes it only when the matching response arrives. Under a burst of N in-flight requests, every worker response is dispatched to all N attached listeners (each then runs an id/method check and returns), producing O(n²) main-thread dispatch work.

This was hit in production by two enterprise customers when Zendesk's web widget started inserting CSS rules at high frequency: rrweb emitted a mutation per rule, Replay forwarded each via postMessage, and the listener fan-out made the page unresponsive for several seconds.

Source: packages/replay-internal/src/eventBuffer/WorkerHandler.ts#L71-L106

public postMessage<T>(method, arg): Promise<T> {
  const id = this._getAndIncrementId();
  return new Promise((resolve, reject) => {
    const listener = ({ data }) => {
      if (response.method !== method) return;
      if (response.id !== id) return;
      this._worker.removeEventListener('message', listener);
      // resolve/reject…
    };
    this._worker.addEventListener('message', listener); // ← N listeners during burst
    this._worker.postMessage({ id, method, arg });
  });
}

Reproduction: https://github.com/logaretm/sentry-replay-worker-quadratic-repro