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/wordpress-develop#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.
Built from https://develop.svn.wordpress.org/trunk@54311


git-svn-id: https://core.svn.wordpress.org/trunk@53870 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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(

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-beta1-54310';
+$wp_version = '6.1-beta1-54311';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.