Leverage scheduler.yield in splitTask when available

[westonruter]

Fixes #64483.

What?

Use scheduler.yield() in the splitTask() function in @wordpress/interactivity when available. Otherwise, continue to fall back to using setTimeout as currently.

Why?

In #58227 a yieldToMain() function was introduced to break up long hydration tasks when a document is initialized by the Interactivity API. At the time, this function was implemented using setTimeout() as shown in the Optimize long tasks article on web.dev. That article also describes a more performant mechanism in scheduler.yield() which would be coming at a later date. That date is now in Chromium browsers (caniuse).

In #62665 the yieldToMain() function was renamed to splitTask() and was then exported by the @wordpress/interactivity package for extensions to leverage (e.g. in Async Actions).

On a post that contains 38 interactive blocks (36 of which are Image blocks with lightboxes) the performance benefits of using scheduler.yield() in the following two traces (both run with 6x CPU slowdown on a HP Dragonfly Elite Chromebook):

1. Scripting time is reduced by 21%.
2. Painting time is reduced by 36%.
3. Idle time is reduced by 57% as is seen in the elimination of gaps in the flame chart.
4. Hydration completes before the load event.

Splitting tasks with setTimeout()

See trace.cafe.

Splitting tasks with scheduler.yield()

See trace.cafe

Additional Observation

When scheduler.yield() is used, I see a 99ms long task caused by flushAfterPaintEffects:

When setTimeout() is used, these tasks are split up (where the black outlines indicate instances of flushAfterPaintEffects):

This appears to be due to the fact that flushAfterPaintEffects itself is scheduled for execution using a timer, meaning that when scheduler.yield() is used, the pending paint effects are allowed to bunch up for one single call. This is part of Preact, and perhaps that long task can be addressed upstream by introducing scheduler.yield() there as well (cf. facebook/react#27069).

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[swissspidy]

This is more readable IMO and avoids an async function in the promise body:

export const splitTask = () => {
	if (
		'scheduler' in window &&
		typeof window.scheduler === 'object' &&
		null !== window.scheduler &&
		'yield' in window.scheduler &&
		typeof window.scheduler.yield === 'function'
	) {
		return window.scheduler.yield();
	}
	
	return new Promise( ( resolve ) => {
		setTimeout( resolve, 0 );
	} );
}

[westonruter]

Yes, perfect. I spaced on this. I got the same feedback from @LeszekSwirski.

[westonruter]

I've applied that in 70077b8 and then further improved it in d5c51a6 by only doing the check for scheduler.yield() once.

[westonruter]

And improved yet further in 2516cd4 by removing the wrapper function around scheduler.yield(). So when scheduler.yield() is available, splitTask is just a reference to that function.

[westonruter]

Humm. Actually this results in an error for some reason:

I guess it's because yield here doesn't have scheduler as its context object.

[westonruter]

Fixed in a0933f4 via bind().

[westonruter]

There's probably zero difference in the end between:

window.scheduler.yield.bind( window.scheduler )

And:

() => {
    return window.scheduler.yield();
}

Since in both cases a new function is involved. The former seems to be syntactic sugar for the latter.

[felixarntz]

But why use bind over the latter? I find the latter more intuitive to understand. Also, I'm not familiar enough with bind to be confident so that is the same. Since you also say "probably", I'd suggest we go with the latter.

We have to use a function anyway for the fallback of returning a promise, so we might as well wrap the entire code in a function.

[LeszekSwirski]

"bind" is very very slightly preferable for the JS engine, since it can represent the result as a "bound function" that forwards directly to the function it binds already while resolving the call, and doesn't need to parse it, compile it, interpret bytecode, etc.

[felixarntz]

Thanks @westonruter!

[youknowriad]

In other places in Gutenberg, I believe we use requestIdleCallback to optimize for UI responsiveness and delay some tasks with lower priority. I wonder if you can speak about the difference and when to favor one over the other.

[westonruter]

With requestIdleCallback the idleness may occur several seconds after requested which would not be desirable for hydration. For splitTask it's purely to break up long tasks which remain high priority. So both have their use cases.

[t-hamano]

This PR may have had an impact on the lightbox. Please see #66691.