Block Supports: Allow CSS custom properties in block gap sanitization

[noruzzamans]

What?

Closes #82747

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

Why?

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

1. gutenberg_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 gutenberg_render_layout_support_flag() are passed through gutenberg_sanitize_block_gap_value().
3. Because gutenberg_sanitize_block_gap_value() disallows parentheses (, ) to prevent CSS injection, the resolved preset was rejected and returned null.
4. As a result, the Grid layout calculation in gutenberg_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, the column calculation formula max(min(..., 100%), (100% - (gap * (n - 1))) / n) was computed against 0.5em, causing grid items to overflow or stack into a single column.

How?

• Updated gutenberg_sanitize_block_gap_value() in lib/block-supports/layout.php to accept valid CSS custom property references matching /^var\(--[a-zA-Z0-9_-]+\)$/.
• Added unit test cases in phpunit/block-supports/layout-test.php covering gutenberg_sanitize_block_gap_value() with valid and invalid CSS variables, including nested array handling.
• Added a test case in data_gutenberg_get_layout_style() verifying that Grid layout with a preset CSS variable fallback correctly computes the grid-template-columns declaration.

Testing Instructions

1. Run the PHPUnit test suite:

   npm run wp-env-test run -- --env-cwd='wp-content/plugins/gutenberg' wordpress vendor/bin/phpunit -c phpunit.xml.dist --filter test_sanitize_block_gap_value
   npm run wp-env-test run -- --env-cwd='wp-content/plugins/gutenberg' wordpress vendor/bin/phpunit -c phpunit.xml.dist --filter test_gutenberg_get_layout_style

2. Verify all tests pass.
3. Test with a block theme setting a preset block gap in theme.json:

   "styles": {
     "spacing": {
       "blockGap": "var:preset|spacing|32"
     }
   }

4. Insert a Grid block with columnCount: 2 and minimumColumnWidth: "20rem".
5. Check the rendered front-end CSS output for the grid container; verify grid-template-columns uses var(--wp--preset--spacing--32) rather than falling back to 0.5em.

Use of AI Tools

AI assistance: Yes
Model(s): Gemini 3.8 Flash High

Assisted by Gemini 3.8 Flash High to investigate the root cause in layout.php and compose the PHPUnit test cases.

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Co-authored-by: noruzzamans <noruzzaman@git.wordpress.org>
Co-authored-by: kadamwhite <kadamwhite@git.wordpress.org>

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