Boost: Improve responsiveness during CCSS generation retry

[haqadn]

Fixes #38169
Closes #40407

Proposed changes:

• Optimistically update the UI to avoid any flicker when critical CSS generation is attempted after an error.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Use the Boost Dev plugin and set "Force Critical CSS generation Error" to timeout.
• Attempt C.CSS generation.
• It should fail, and you should see an error.
• Retry the generation.
• It should show the generating progress bar immediately without the UI glitch seen on Boost: Error UI takes a while to update after refreshing #38169
• Make sure cloud CSS and local C.CSS are both working correctly in different situations after this change.

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[dilirity]

Free critical css seems to work fine:

xXgEoRC2B1ZctQpX8yaQ04Crur7SAhCItwLztQGR.mp4

Although, I noticed the Contact Support button flashing for a brief moment when you click refresh.

I recorded a video of the above recording and slowed it down manually. I think it's just one frame when re-rendering the component:

LVwsou3nDVTZdckac27aLV0gKGlolK4yrL6Z4NLO.mp4

Just wanted to add this, technically the user will miss it, but it seems something's a miss. I don't think it should be happening.

For easier testing the free version, you can use this - just one page in the payload:

add_filter( 'jetpack_boost_critical_css_providers', function ( $sources ) {
	return array(
		array (
			'key' => 'core_posts_page',
			'label' => 'Posts page',
			'urls' => array (
				'http://jetpack-boost.test/blog/',
			),
			'success_ratio' => 1,
		),
	);
} );

For premium, there's still an issue. It's hard to demonstrate, but you can try it for yourself by throttling the network before clicking refresh.

Here's what it looks like:

eTEWMdr0AJmZTjqms9JlZ9L5r5ejdYITE0IFpey4.mp4

Clicking refresh immediately shows the Contact Support button instead of the progress description.

[haqadn]

• I had inadvertently broken the retried state while working on this. A fix for that was f899ee1.
• Addressed the cloud CSS issue be17aa6.
• The single frame issue: bd531b1. Because we are in the business of improving web vitals after all.

[dilirity]

Wow - awesome work on this!

I couldn't find issues with the UI.