Theme JSON: Ensure root selector (body) is not wrapped in :root :where().

Pre-WordPress 6.6, the `body` selector was used for styles associated with the body.

In 6.6, this was mistakenly changed to `:root :where(body)`, an increase in specificity, causing some issues for themes.

This change reverts the specificity increase, styles again use the `body` selector.

Syncs PHP changes from https://github.com/WordPress/gutenberg/pull/63726.

Reviewed by hellofromTonya.
Merges [58856] and [58861] to the 6.6 branch.

Props talldanwp, andrewserong, aaronrobertshaw, mukesh27, hellofromTonya.
Fixes #61704.
Built from https://develop.svn.wordpress.org/branches/6.6@58874


git-svn-id: http://core.svn.wordpress.org/branches/6.6@58270 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-theme-json.php

@@ -2880,6 +2880,9 @@ class WP_Theme_JSON {
 		}
 
 		/*
+		 * Root selector (body) styles should not be wrapped in `:root where()` to keep
+		 * specificity at (0,0,1) and maintain backwards compatibility.
+		 *
 		 * Top-level element styles using element-only specificity selectors should
 		 * not get wrapped in `:root :where()` to maintain backwards compatibility.
 		 *
@@ -2887,11 +2890,13 @@ class WP_Theme_JSON {
 		 * still need to be wrapped in `:root :where` to cap specificity for nested
 		 * variations etc. Pseudo selectors won't match the ELEMENTS selector exactly.
 		 */
-		$element_only_selector = $current_element &&
+		$element_only_selector = $is_root_selector || (
+			$current_element &&
 			isset( static::ELEMENTS[ $current_element ] ) &&
 			// buttons, captions etc. still need `:root :where()` as they are class based selectors.
 			! isset( static::__EXPERIMENTAL_ELEMENT_CLASS_NAMES[ $current_element ] ) &&
-			static::ELEMENTS[ $current_element ] === $selector;
+			static::ELEMENTS[ $current_element ] === $selector
+		);
 
 		// 2. Generate and append the rules that use the general selector.
 		$general_selector = $element_only_selector ? $selector : ":root :where($selector)";

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6.2-alpha-58869';
+$wp_version = '6.6.2-alpha-58874';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.