Block Supports: Prevent Additional CSS duplication inside Query Loop

[mustafabharmal]

When using the Additional CSS feature on a block inside a Query Loop, the generated CSS is inserted once for every rendered post/item in the loop. This causes the same CSS rules to be duplicated N times in the page output.

Root cause: gutenberg_render_custom_css_support_styles is hooked to render_block_data, which fires once per block render. Inside a Query Loop the same block template renders once per post, so wp_add_inline_style is called N times with the same CSS content.

Since wp_unique_id_from_values produces the same hash for the same block data, each loop iteration generates the same $class_name and the same $processed_css.

Fix: Use a static variable to track already-enqueued class names so subsequent loop iterations skip the redundant wp_add_inline_style call.

Gutenberg issue: ​https://github.com/WordPress/gutenberg/issues/78278
Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/78282

Trac ticket: https://core.trac.wordpress.org/ticket/65268

## What
When a block inside a Query Loop has Additional CSS, wp_add_inline_style was called once per post in the loop, duplicating the same CSS N times.

## Why
wp_render_custom_css_support_styles hooks render_block_data, which fires for every block render. wp_unique_id_from_values produces the same hash for the same block template data on every loop iteration, so the same CSS gets injected repeatedly.

## How
Added a static $enqueued_class_names array. The first time a class name is seen the CSS is enqueued; subsequent renders skip it.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/78282

[audrasjb]

Removing trunk version as this is not going to be shipped with WP 7.0 but in the next releases.

[desrosj]

I'm unclear if this is a regression in 7.0 or not. Leaving trunk as the version for now.

We should add a test for this as well.

When a block inside a Query Loop has Additional CSS, wp_add_inline_style was called once per post in the loop, duplicating the same CSS N times. This was because wp_unique_id_from_values produces the same hash for the same block data on every loop iteration.

This fix adds a static $enqueued_class_names array to track already-enqueued class names so subsequent loop iterations skip the redundant wp_add_inline_style call.

Props mustafabharmal, westonruter.
Fixes #65268.

Trac ticket:

## Use of AI Tools

Thank you for the PR. However, we already have an ongoing PR for this: ​https://github.com/WordPress/wordpress-develop/pull/11859

@Mustafabharmal Could you add some unit tests for this PR please? After the commit we can check as well if the failing check is working again. We'd like to get this a step further to potentially include it in WP 7.0.1

Thanks for your help here!

[cbravobernal]

Punting to 7.1

LGTM! I would like to commit this later.

Go for it.

@pento, it appears that unnecessary changes were added due to the force push.

@pento, it appears that unnecessary changes were added due to the force push.

+1

I'm not entirely sure of the cause, but when I checked out the latest trunk locally and rebased this branch on top of it, the unintended changes seem to have disappeared.

[wildworks]

In 62656:

Block Supports: Fix duplicate custom CSS for identical attributes.

wp_render_custom_css_support_styles() runs on every block render, and wp_unique_id_from_values() derives the class name from the block's attributes. Blocks with identical attributes therefore share the same processed CSS, which wp_add_inline_style() re-enqueued on each render, emitting the same rule once per occurrence.

Before enqueuing, skip the CSS if it is already present in the wp-block-custom-css handle's after inline styles`.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/11859.

Props masteradhoc, mukesh27, mustafabharmal, westonruter, wildworks.
Fixes #65268.