React effect cleanup missing

A production symptom appears first: flaky tests, stale state, slow UI, late cleanup, or growing memory. This appears in dashboards, resize measurement, polling widgets, analytics listeners, and components that mount/unmount often.

Assuming React automatically clears timers and browser listeners.

Fix the root scheduling, cleanup, or concurrency pattern instead of adding arbitrary waits.

try {
  await operation();
} finally {
  cleanup();
}

The console lane proves the visible order: still syncing. The active line and queues show why source order changed.

Reproduce the symptom with a small test, apply the safer pattern, then verify the flaky, slow, or leaking behavior disappears.