Editor: Fix enqueueing additional styles in wp_enqueue_block_style() to print only when blocks render.

In a block theme, additional block styles registered using the `wp_enqueue_block_style` function should only get printed when the block exists on a page. However, they currently always get rendered.

This commit is a backport from Gutenberg that fixes the issue by printing the styles when a block renders.

Follow-up to [52069].

Props poena, aristath, Mamaduka.
Fixes #54787.

git-svn-id: https://develop.svn.wordpress.org/trunk@52556 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks.php

@@ -1254,7 +1254,31 @@ function wp_enqueue_block_style( $block_name, $args ) {
 
 	$hook = did_action( 'wp_enqueue_scripts' ) ? 'wp_footer' : 'wp_enqueue_scripts';
 	if ( wp_should_load_separate_core_block_assets() ) {
-		$hook = "render_block_$block_name";
+		/**
+		 * Callback function to register and enqueue styles.
+		 *
+		 * @param string $content The block content.
+		 * @param array  $block   The full block, including name and attributes.
+		 * @return string Block content.
+		 */
+		$callback_separate = static function( $content, $block ) use ( $block_name, $callback ) {
+			if ( ! empty( $block['blockName'] ) && $block_name === $block['blockName'] ) {
+				return $callback( $content );
+			}
+			return $content;
+		};
+
+		/*
+		 * The filter's callback here is an anonymous function because
+		 * using a named function in this case is not possible.
+		 *
+		 * The function cannot be unhooked, however, users are still able
+		 * to dequeue the stylesheets registered/enqueued by the callback
+		 * which is why in this case, using an anonymous function
+		 * was deemed acceptable.
+		 */
+		add_filter( 'render_block', $callback_separate, 10, 2 );
+		return;
 	}
 
 	/*