What problem does this feature solve?

Nowadays, with the drop of IE support, queueMicrotask is available everywhere, and has been for several years.

It doesn't change much, but would it be a tiny bit simpler and shorter to use it in scheduler.ts rather than juggle Promises?

What does the proposed API look like?

This is a minor implementation detail change, no public API change.

Vue 3 runs in Safari 10+, but Safari only supports queueMicrotask in 12.2+. There are most likely other mismatches between the ES2015 support bottom line and the availability of queueMicrotask. Moving to this natively would be a breaking change, and falling back to native doesn't seem to provide any real value since we need to keep shipping the polyfill anyway.

Fair, I was wary of phones holding you back!

Out of curiosity, in this day of "evergreen" browsers, what is the Vue strategy for using web tech in years to come?

Is there an official list of minimally supported version that you will bump from time to time? Will you refrain from using new apis until a big new Vue 4 comes along in a few years?

An idea could be to have an evergreen Vue and a list of polyfill requirements for those targeting old phones or other constrained environments?