MutationObserver timing

A production symptom appears first: flaky tests, stale state, slow UI, late cleanup, or growing memory. This matters in component libraries, rich text editors, analytics trackers, test utilities, and DOM measurement code.

Assuming DOM observers behave like setTimeout callbacks.

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: sync -> observer -> timer. 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.