Theme JSON: replace top-level background style objects on merge

This commit fixes an omission in the logic of `WP_Theme_JSON::merge()` where top-level background image style objects are not replaced, rather they are merged. Because background images are self contained objects, their properties are unique and should not be merged. Blocks are already catered for via `WP_Theme_JSON::get_block_nodes()`.

Follow-up to [61858].

Props ramonopoly, andrewserong.
Fixes #62328.




git-svn-id: https://develop.svn.wordpress.org/trunk@59335 602fd350-edb4-49c9-b593-d223f7449a82

Author: ramonjd

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

@@ -3268,6 +3268,10 @@ public function merge( $incoming ) {
 			array(),
 			array( 'include_node_paths_only' => true )
 		);
+
+		// Add top-level styles.
+		$style_nodes[] = array( 'path' => array( 'styles' ) );
+
 		foreach ( $style_nodes as $style_node ) {
 			$path = $style_node['path'];
 			/*

tests/phpunit/tests/theme/wpThemeJson.php

@@ -2344,7 +2344,9 @@ public function test_merge_incoming_background_styles() {
 				'styles'  => array(
 					'background' => array(
 						'backgroundImage' => array(
-							'url' => 'http://example.org/quote.png',
+							'id'     => 'uploaded',
+							'source' => 'file',
+							'url'    => 'http://example.org/quote.png',
 						),
 						'backgroundSize'  => 'cover',
 					),
@@ -2376,7 +2378,10 @@ public function test_merge_incoming_background_styles() {
 			'version' => WP_Theme_JSON::LATEST_SCHEMA,
 			'styles'  => array(
 				'background' => array(
-					'backgroundSize' => 'contain',
+					'backgroundImage' => array(
+						'url' => 'http://example.org/site.png',
+					),
+					'backgroundSize'  => 'contain',
 				),
 				'blocks'     => array(
 					'core/group' => array(
@@ -2406,7 +2411,7 @@ public function test_merge_incoming_background_styles() {
 			'styles'  => array(
 				'background' => array(
 					'backgroundImage' => array(
-						'url' => 'http://example.org/quote.png',
+						'url' => 'http://example.org/site.png',
 					),
 					'backgroundSize'  => 'contain',
 				),