Automatic Batching Divergence After Failed Deployments in React 18

Direct answer

Non-batched updates after a failed deployment come from state changes scheduled outside React's synthetic event system: promise resolution callbacks, setTimeout and setInterval handlers, native addEventListener listeners, and third-party libraries that call dispatchEvent or invoke setters directly on the DOM. React 18 automatic batching covers updates inside React event handlers attached through JSX; it does not cover those asynchronous or native paths. During rollback, if a promise resolves and updates deployment status while a separate UI slice is still rendering from a prior state, the commits can interleave and leave the shell and content diverged.

Likely explanation:

The deployment failure triggers rollback logic in a promise chain or third-party callback. Those updates render outside React's batching window, so the UI commits partial rollback state before dependent state has settled.

Confirmed facts for React 18:

• Updates inside React synthetic events are batched automatically when using ReactDOM.createRoot.
• Updates in promises, timeouts, and native event listeners are not automatically batched.
• flushSync forces a synchronous render commit for the wrapped update.
• unstable_batchedUpdates from react-dom can wrap multiple updates so they commit together.

Steps for this case

1. Find every deployment and rollback state update. Mark whether it runs inside a React handler, a promise callback, a timer, or a third-party listener.
2. Wrap the critical rollback transition in flushSync only if the UI must reflect the failed-deployment state before any other render occurs. Treat this as a performance exception, not a default.
3. Wrap multi-step rollback sequences in unstable_batchedUpdates so related state slices commit in one render.
4. Replace native listeners around deployment status with React synthetic handlers, or wrap third-party callbacks so their updates enter through React's batching boundary.
5. Verify with React DevTools Profiler during a throttled failed-deploy simulation; add short-lived console logs around each deployment state update to confirm which path fired.

Missing diagnostic detail

Are the deployment and rollback updates triggered inside React synthetic event handlers, or from promise timers and native listeners? This determines whether automatic batching already applies or whether flushSync or unstable_batchedUpdates is required.

Assumptions and uncertainty

This assumes React 18 with ReactDOM.createRoot. React batching semantics can change in future releases, and third-party library behavior varies. Verify the installed React version and inspect third-party event handling before applying flushSync broadly.