163845 2016-10-22 04:01:22 -0700 Promise - MutationObserver interaction spins the event loop 2019-11-22 22:33:58 -0800 1 1 1 Unclassified WebKit JavaScriptCore Safari Technology Preview Mac OS X 10.11 RESOLVED WORKSFORME https://bugs.webkit.org/show_bug.cgi?id=204546 P2 Normal --- 1 miklos.bertalan webkit-unassigned ap cdumez rniwa youennf ysuzuki oldest_to_newest 1243357 0 miklos.bertalan 2016-10-22 04:01:22 -0700 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. 1243358 1 292494 miklos.bertalan 2016-10-22 04:02:26 -0700 Created attachment 292494 Script to recreate the bug 1593181 2 rniwa 2019-11-22 22:33:51 -0800 This appears to be fixed now. 292494 2016-10-22 04:02:26 -0700 2016-10-22 04:02:26 -0700 Script to recreate the bug PromiseMutObserverInteraction.html text/html 524 miklos.bertalan PGRpdiBjbGFzcz0idGVzdGVyIj48L2Rpdj4KPHNjcmlwdD4KY29uc3QgdGVzdGVyID0gZG9jdW1l bnQucXVlcnlTZWxlY3RvcignLnRlc3RlcicpCmxldCBjb3VudGVyID0gMAoKc2V0VGltZW91dCgo KSA9PiBjb25zb2xlLmxvZygndGltZW91dCcpKQpyZXF1ZXN0QW5pbWF0aW9uRnJhbWUoKCkgPT4g Y29uc29sZS5sb2coJ2FuaW1hdGlvbkZyYW1lJykpCgpuZXcgTXV0YXRpb25PYnNlcnZlcigoKSA9 PiB7CiAgY29uc29sZS5sb2coJ211dGF0aW9uJykKICBpZiAoY291bnRlciA8IDEwMCkgewogICAg UHJvbWlzZS5yZXNvbHZlKCkudGhlbigoKSA9PiB7CiAgICAgIGNvbnNvbGUubG9nKCdwcm9taXNl JykKICAgICAgdGVzdGVyLnNldEF0dHJpYnV0ZSgnZGF0YS1yYW5kb20nLCBNYXRoLnJhbmRvbSgp KQogICAgfSkKICB9CiAgY291bnRlcisrCn0pLm9ic2VydmUodGVzdGVyLCB7YXR0cmlidXRlczog dHJ1ZX0pCgp0ZXN0ZXIuc2V0QXR0cmlidXRlKCdkYXRhLXJhbmRvbScsIE1hdGgucmFuZG9tKCkp Cjwvc2NyaXB0Pgo=