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<base href="https://bugs.webkit.org/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Promise - MutationObserver interaction spins the event loop"
href="https://bugs.webkit.org/show_bug.cgi?id=163845">163845</a>
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</tr>
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<th>Summary</th>
<td>Promise - MutationObserver interaction spins the event loop
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr>
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<th>Product</th>
<td>WebKit
</td>
</tr>
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<th>Version</th>
<td>Safari Technology Preview
</td>
</tr>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>OS X 10.11
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>Normal
</td>
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<th>Priority</th>
<td>P2
</td>
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<th>Component</th>
<td>JavaScriptCore
</td>
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<th>Assignee</th>
<td>webkit-unassigned@lists.webkit.org
</td>
</tr>
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<th>Reporter</th>
<td>miklos.bertalan@risingstack.com
</td>
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<p>
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<pre>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.</pre>
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