Intent to implement & ship: queueMicrotask

[smaug]

Summary: queueMicrotask exposes the platform primitive to queue microtasks. Without the API, web pages have used hacks around MutationObserver or Promise.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1480236
Link to standard: https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#microtask-queuing
Platform coverage: everywhere
Estimated or target release: 69
Preference behind which this will be implemented: without pref
Is this feature enabled by default in sandboxed iframes? yes
DevTools bug: NA
Do other browser engines implement this? Shipped in Blink and Webkit
web-platform-tests:
https://searchfox.org/mozilla-central/source/testing/web-platform/tests/html/webappapis/microtask-queuing
Is this feature restricted to secure contexts? No. This is just exposing the scheduling primitive of MutationObserver and Promise as an API.




Note to the users of the API (this applies to Promises and MutationObserver too). Microtasks are from UA point of view synchronous, so don't do heavy
operations in them, or schedule lots of them in a row - that might cause jank.





-Olli