Add comments to the server side rendering functions.

packages/block-library/src/accordion-group/index.php

@@ -10,7 +10,6 @@
  *
  * @return string Returns the updated markup.
  */
-
 function render_block_core_accordion_group( $attributes, $content ) {
 	if ( ! $content ) {
 		return $content;
@@ -45,10 +44,18 @@ function render_block_core_accordion_group( $attributes, $content ) {
 	return $content;
 }
 
+/**
+ * Returns the existing list of ids, or adds a new id to the list.
+ *
+ * @since 6.6.0
+ *
+ * @param string $id The id of the accordion item.
+ * @return array|void Returns the ids for the  or nothing.
+ */
 function block_core_accordion_group_item_ids( $id = null ) {
 	static $ids = array();
 	if ( null === $id ) {
-		// Returns the ids and reset them for the next accordion group.
+		// Returns the ids and resets them for the next accordion group.
 		$current_ids = $ids;
 		$ids         = array();
 		return $current_ids;

packages/block-library/src/accordion-item/index.php

@@ -19,12 +19,15 @@ function block_core_accordion_item_render( $attributes, $content ) {
 	$p         = new WP_HTML_Tag_Processor( $content );
 	$unique_id = wp_unique_id( 'accordion-item-' );
 
-	// Adds the id to the current accordion group list.
+	// If the item is open by default,
+	// add its id to the accordion group's initial context.isOpen list.
 	if ( $attributes['openByDefault'] ) {
 		// phpcs:ignore Gutenberg.CodeAnalysis.ForbiddenFunctionsAndClasses.ForbiddenFunctionCall
 		gutenberg_block_core_accordion_group_item_ids( $unique_id );
 	}
 
+	// Initialize the state of the item on the server using a closure,
+	// since we need to get derived state based on the current context.
 	wp_interactivity_state(
 		'core/accordion',
 		array(
@@ -43,14 +46,14 @@ function block_core_accordion_item_render( $attributes, $content ) {
 			$p->set_attribute( 'data-wp-on--click', 'actions.toggle' );
 			$p->set_attribute( 'aria-controls', $unique_id );
 			$p->set_attribute( 'data-wp-bind--aria-expanded', 'state.isOpen' );
-		}
 
-		if ( $p->next_tag( array( 'class_name' => 'wp-block-accordion-content' ) ) ) {
-			$p->set_attribute( 'aria-labelledby', $unique_id );
-			$p->set_attribute( 'data-wp-bind--aria-hidden', '!state.isOpen' );
+			if ( $p->next_tag( array( 'class_name' => 'wp-block-accordion-content' ) ) ) {
+				$p->set_attribute( 'aria-labelledby', $unique_id );
+				$p->set_attribute( 'data-wp-bind--aria-hidden', '!state.isOpen' );
 
-			// Only modify content if all directives have been set.
-			$content = $p->get_updated_html();
+				// Only modify content if all directives have been set.
+				$content = $p->get_updated_html();
+			}
 		}
 	}