X_PWA_SYNC_FLUSH_FAILED

the background-sync outbox flush was rejected

registered, thrown by nothing since 21.0.0. Until then the worker POSTed /_x/outbox/flush, a route nothing mounted, and raised this on a non-2xx. The worker now posts OUTBOX_DRAIN_MESSAGE to open tabs, and realtime's outbox replays over HTTP

delete any branch that matches this code, because it never runs. An app still generating the old worker: delete backgroundSync: { flushEndpoint } from its generateServiceWorker call