Switch to MacrotaskExecutor on JS

[armanbilge]

Closes #1519. Targeting series/3.x.

See https://github.com/scala-js/scala-js-macrotask-executor for more details.

Some notes:

1. This required bumping to Scala.js 1.7.1.
2. The monix-internal StandardContext this replaces actually did not suffer from the fairness problem that the default global ExecutionContext in Scala.js does. However, it seems like it would suffer from serious performance issues when used in browsers. Is anyone aware of this?

[armanbilge]

The MacrotaskExecutor is basically an advanced version of this conditional. First it looks for setImmediate (best performance). However, if that's not available, instead of giving up and falling back to setTimeout (very bad performance), MacrotaskExecutor attempts to implement a number of higher-performance polyfills instead.

[armanbilge]

2. The monix-internal StandardContext this replaces actually did not suffer from the fairness problem that the default global ExecutionContext in Scala.js does. However, it seems like it would suffer from serious performance issues when used in browsers. Is anyone aware of this?

Yes, according to scala-js/scala-js#4129 (comment):

Monix has been using setImmediate in its Scheduler.global whenever available, ever since I ported it to Scala.js. Nobody complained.

[alexandru]

I was thinking of doing window.postMessage, not sure if feasible

#1556

[alexandru]

I took the liberty of merging with the base branch, and doing some fixes in build.sbt and in that test.

Note that in Monix I'd prefer to drop 3rd party dependencies in the core modules, however seeing this is an "official" Scala.js project, I guess it's all good.

Thanks for the PR. Will merge as soon as the build passes.

[armanbilge]

Note that in Monix I'd prefer to drop 3rd party dependencies in the core modules, however seeing this is an "official" Scala.js project, I guess it's all good.

Yes, I read your blog post about the CE2/CE3 thing and 👍

The MacrotaskExecutor project should be very stable, ideally as stable as Scala.js itself. It's not part of Scala.js core largely due to their own policy/preference to avoid polyfills and non-standard APIs in core.