Idle callback background work

A production symptom appears first: flaky tests, stale state, slow UI, late cleanup, or growing memory. This helps with analytics flushing, precomputing, non-urgent persistence, and performance-friendly cleanup.

Doing analytics, cache warming, or cleanup on the critical interaction path.

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: paint important UI -> send analytics. 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.