Block Supports: Allow CSS custom properties in wp_sanitize_block_gap_value()

[noruzzaman]

When a theme defines a preset block gap in theme.json (such as styles.spacing.blockGap: "var:preset|spacing|32"):

1. wp_get_global_styles() resolves the preset to its CSS variable format: var(--wp--preset--spacing--32).
2. Candidate values for the global block gap in layout rendering are passed through wp_sanitize_block_gap_value().
3. Because wp_sanitize_block_gap_value() disallows parentheses to prevent CSS injection, the resolved preset is rejected and returns null.
4. As a result, Grid layout calculations in wp_get_layout_style() fall back to 0.5em instead of using the theme's preset gap value.
5. In Grid blocks with columnCount and minimumColumnWidth, columns stack into a single column.

Fix: Allow valid CSS custom property references matching /^var\(--[a-zA-Z0-9_-]+\)$/.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/82817

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/82817

Description

Allows valid CSS custom property references (such as var(--wp--preset--spacing--32)) in wp_sanitize_block_gap_value().

When a theme defines a preset block gap in theme.json (such as styles.spacing.blockGap: "var:preset|spacing|32"):

1. wp_get_global_styles() resolves the preset to its CSS variable format: var(--wp--preset--spacing--32).
2. Candidate values for the global block gap in layout rendering are passed through wp_sanitize_block_gap_value().
3. Because wp_sanitize_block_gap_value() disallows parentheses to prevent CSS injection, the resolved preset was rejected and returned null.
4. As a result, Grid layout calculations in wp_get_layout_style() fell back to 0.5em instead of using the theme's preset gap value.
5. In Grid blocks with columnCount and minimumColumnWidth, columns stack into a single column.

Testing Instructions

1. Run PHPUnit test suite:

   npm run test:php -- tests/phpunit/tests/block-supports/layout.php
   

2. Verify all tests pass including CSS variable preset sanitization and grid layout fallback.