Editor: Fix link color CSS cascade conflict for duplicate blocks with identical markup

[USERSATOSHI]

Trac ticket: https://core.trac.wordpress.org/ticket/65435

Description

This fixes a CSS cascade bug where identical block content produces duplicate wp-elements-* class names, causing parent block element styles (e.g. link colors) to incorrectly override child block styles.

Root cause

wp_get_elements_class_name() generates CSS class names via md5( serialize( $block ) ).
When two blocks have identical parsed data (e.g. two Paragraph blocks with the same text, one standalone and one inside a Group), they produce the same class.
The Style Engine deduplicates the rule, and since the parent Group's rule renders later in DOM order, it overrides the child Paragraph's style.

A contributing factor: elements.php is required before layout.php in wp-settings.php, so wp_render_elements_support_styles() runs before wp_add_parent_layout_to_parsed_block().
If layout ran first, nested blocks would have a parentLayout key, producing different hashes.

Fix

Introduces a collision tracker via static $seen_hashes in wp_get_elements_class_name(). When a hash collision is detected, an incrementing counter is appended before re-hashing, producing a unique class name per duplicate instance. This ensures the Style Engine outputs distinct CSS rules for each block instance, preserving correct cascade order.

Testing

Added Test cases for these cases.

• tests/phpunit/tests/block-supports/wpRenderElementsSupportStyles.php::test_elements_block_support_styles_with_duplicate_blocks
• tests/phpunit/tests/block-supports/wpRenderElementsSupport.php::test_elements_block_support_class_with_duplicate_blocks

Screenshots

Before	After

Credits

• @dpmehta: Initial investigation, root cause analysis, and Gutenberg fix prototype.

References

• Gutenberg issue: Link color is not applied correctly on front end gutenberg#53803 (Link color is not applied correctly on front end)

Use of AI Tools

AI assistance: Yes
Tool(s): OpenCode
Model(s): deepseek-v4-flash-free
Used for: I used AI only for creating the test scaffolding. Code is reviewed and written by me.

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