Custom CSS: Prevent duplicate custom css styles

[aaronrobertshaw]

What?

Removes core WordPress custom CSS block support filters and action before registering Gutenberg's replacements.

Why?

PR #75799 introduced Gutenberg-prefixed versions of the custom CSS block support functions (gutenberg_render_custom_css_support_styles, gutenberg_render_custom_css_class_name, gutenberg_enqueue_block_custom_css), but did not remove the corresponding core WordPress hooks (wp_render_custom_css_support_styles, wp_render_custom_css_class_name, wp_enqueue_block_custom_css). This results in both the core and Gutenberg versions running, producing duplicate custom CSS styles on the frontend.

How?

Before registering the Gutenberg filters and action, remove the core equivalents using remove_filter / remove_action, guarded by function_exists() checks for backward compatibility with older WordPress versions where these functions may not exist.

This follows the established pattern used by other block supports in this directory, e.g., block-style-variations.php, elements.php, layout.php, and duotone.php.

Testing Instructions

1. Activate the Gutenberg plugin.
2. Add a block (e.g., a Paragraph block) with custom CSS via the block's "Additional CSS" panel in the editor.
3. View the page on the frontend.
4. Inspect the page source — confirm the custom CSS styles appear only once, not duplicated.

Screenshots

Before	After

[im3dabasia]

LGTM ✅

Thanks for the PR! Works as expected. In my testing, the second CSS snippet turned out to be redundant and wasn't being used anywhere. It also tests well against the global styles.

Before	After

[github-actions]

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Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>

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[andrewserong]

LGTM, too 👍

[ramonjd]

Good spotting! 🙇🏻

[talldan]

I noticed there was also an issue (#75828) which I've closed as completed.