Bug 163845

Queueing a Promise from a MutationObserver handler or triggering an observed change from a Promise callback spins the event loop. According to the spec, it should register a new microtask and not a new task. Code to reproduce (it is also attached): <div class="tester"></div> <script> const tester = document.querySelector('.tester') let counter = 0 setTimeout(() => console.log('timeout')) new MutationObserver(() => { console.log('mutation') if (counter < 100) { Promise.resolve().then(() => { console.log('promise') tester.setAttribute('data-random', Math.random()) }) } counter++ }).observe(tester, {attributes: true}) tester.setAttribute('data-random', Math.random()) </script> Issue: It the above code should print: mutation, promise, mutation, promise, ... timeout Instead it prints: mutation, timeout, promise, mutation, promise, mutation, ... Notes: It seems to get it wrong consistently (not just some of the time). I think it is not caused by any kind of throttling. It seems to queue a new task for both directions (Promise -> Mutation, Mutation -> Promise). (Promise -> Promise) and (Mutation -> Mutation) are both handled correctly as new microtasks.