Update custom CSS handling to be consistent with block global styles.

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/61395

Syncs changes from WordPress/gutenberg#62357.

Steps for testing:

1. Test with custom CSS added at base level and at block level, in theme.json or through the global styles UI.
2. With TT4 enabled, check that custom CSS for blocks is loaded in the front end only when the block is present on the page;
3. Add add_filter( 'should_load_separate_core_block_assets', '__return_false', 11 ); to the theme's functions.php and check that custom CSS for all blocks always loads in the front end.

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[github-actions]

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[github-actions]

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Props isabel_brison, aaronrobertshaw, andrewserong.

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[aaronrobertshaw]

Thanks for putting this together @tellthemachines 👍

✅ Code changes align with Gutenberg PR
✅ With separate block assets loading, custom css is only loaded when block is present
✅ Without separate block assets loading, custom css is always loaded
✅ Same results when testing custom css via global styles or theme.json

LGTM ✨

[andrewserong]

Code-wise this is looking good to me, too. The one difference I noticed while testing is that with a classic theme active, it appears that the customizer's Additional CSS doesn't update in real-time with this PR applied.

Can anyone reproduce this issue, or is it just my environment?

Trunk

2024-07-10.11.34.59.mp4

This PR

2024-07-10.11.37.02.mp4

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This feature is otherwise testing well for me in block themes, the site editor and post editor, and with the styles being output correctly on the site frontend.

[tellthemachines]

Thanks for reviewing, folks!

Can anyone reproduce this issue, or is it just my environment?

I could reproduce that locally; it was due to a missing check that made sure dequeuing customizer CSS only happened for block themes. I've replaced the check now so things should work exactly as they do in trunk.

[tellthemachines]

I've just put up a PR to fix this in Gutenberg too: WordPress/gutenberg#63331

[aaronrobertshaw]

Thanks for the quick fix 🚀

I could replicate the issue and can confirm the latest tweak in 56d5448 fixes it.

Before	After
Screen.Recording.2024-07-10.at.2.50.24.PM.mp4	Screen.Recording.2024-07-10.at.2.48.52.PM.mp4

[andrewserong]

That's testing nicely for me in classic themes now, while block themes and custom CSS continue to work as expected. Thanks for the follow-up!

[tellthemachines]

Thanks for re-testing! Committed in r58703,

[noisysocks]

Thanks for committing this early in the 6.7 cycle ❤️