Editor: Fix spacing property generation in flow layout type.

Fixes a bug of invalid CSS value when applying block spacing to a block as reported in [WordPress/gutenberg#44435 Gutenberg issue 44435].

Adds logic to convert preset values (i.e. `$gap_value`) into valid CSS custom properties for the flow ('default') layout type. See the original fix in [WordPress#3324 Gutenberg PR 3324].

Also adds a test dataset that fails before the bugfix and passes after the bugix.

Follow-up to [54274].

Props ndiego, isabel_brison, ramonopoly, andrewserong, hellofromTonya.
See #56467.

git-svn-id: https://develop.svn.wordpress.org/trunk@54311 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-supports/layout.php

@@ -56,6 +56,13 @@ function wp_get_layout_style( $selector, $layout, $has_block_gap_support = false
 				$gap_value = isset( $gap_value['top'] ) ? $gap_value['top'] : null;
 			}
 			if ( null !== $gap_value && ! $should_skip_gap_serialization ) {
+				// Get spacing CSS variable from preset value if provided.
+				if ( is_string( $gap_value ) && str_contains( $gap_value, 'var:preset|spacing|' ) ) {
+					$index_to_splice = strrpos( $gap_value, '|' ) + 1;
+					$slug            = _wp_to_kebab_case( substr( $gap_value, $index_to_splice ) );
+					$gap_value       = "var(--wp--preset--spacing--$slug)";
+				}
+
 				array_push(
 					$layout_styles,
 					array(

tests/phpunit/tests/block-supports/wpGetLayoutStyle.php

@@ -248,6 +248,20 @@ public function data_wp_get_layout_style() {
 				),
 				'expected_output' => '.wp-layout{flex-wrap:nowrap;flex-direction:column;align-items:flex-start;}',
 			),
+			'default layout with blockGap to verify converting gap value into valid CSS' => array(
+				'args'            => array(
+					'selector'              => '.wp-block-group.wp-container-6',
+					'layout'                => array(
+						'type' => 'default',
+					),
+					'has_block_gap_support' => true,
+					'gap_value'             => 'var:preset|spacing|70',
+					'block_spacing'         => array(
+						'blockGap' => 'var(--wp--preset--spacing--70)',
+					),
+				),
+				'expected_output' => '.wp-block-group.wp-container-6 > *{margin-block-start:0;margin-block-end:0;}.wp-block-group.wp-container-6.wp-block-group.wp-container-6 > * + *{margin-block-start:var(--wp--preset--spacing--70);margin-block-end:0;}',
+			),
 		);
 	}
 }