Make layout supports compatible with enhanced pagination

src/wp-includes/block-supports/layout.php

@@ -630,7 +630,16 @@ function wp_render_layout_support_flag( $block_content, $block ) {
 
 	$class_names        = array();
 	$layout_definitions = wp_get_layout_definitions();
-	$container_class    = wp_unique_id( 'wp-container-' );
+
+	/*
+	 * We use an incremental ID that is independent per prefix to make sure that
+	 * rendering different numbers of blocks doesn't affect the IDs of other
+	 * blocks. We need this to make the CSS class names stable across paginations
+	 * for features like the enhanced pagination of the Query block.
+	*/
+	$container_class = wp_unique_prefixed_id(
+		'wp-container-' . sanitize_title( $block['blockName'] ) . '-layout-'
+	);
 
 	// Set the correct layout type for blocks using legacy content width.
 	if ( isset( $used_layout['inherit'] ) && $used_layout['inherit'] || isset( $used_layout['contentSize'] ) && $used_layout['contentSize'] ) {

src/wp-includes/functions.php

@@ -7830,6 +7830,28 @@ function wp_unique_id( $prefix = '' ) {
 	return $prefix . (string) ++$id_counter;
 }
 
+/**
+ * Generates an incremental ID that is independent per each different prefix.
+ *
+ * It is similar to `wp_unique_id`, but each prefix has its own internal ID
+ * counter to make each prefix independent from each other. The ID starts at 1
+ * and increments on each call. The returned value is not universally unique,
+ * but it is unique across the life of the PHP process and it's stable per
+ * prefix.
+ *
+ * @since 6.4
+ *
+ * @param string $prefix Optional. Prefix for the returned ID. Default empty string.
+ * @return string Incremental ID per prefix.
+ */
+function wp_unique_prefixed_id( $prefix = '' ) {
+	static $id_counters = array();
+	if ( ! array_key_exists( $prefix, $id_counters ) ) {
+		$id_counters[ $prefix ] = 0;
+	}
+	return $prefix . (string) ++$id_counters[ $prefix ];
+}
+
 /**
  * Gets last changed date for the specified cache group.
  *

tests/phpunit/tests/blocks/render.php

@@ -218,9 +218,9 @@ public function test_do_block_output( $html_filename, $server_html_filename ) {
 
 		$html = do_blocks( self::strip_r( file_get_contents( $html_path ) ) );
 		// If blocks opt into Gutenberg's layout implementation
-		// the container will receive an added classname of `wp_unique_id( 'wp-container-' )`
+		// the container will receive an additional, unique classname based on "wp-container-[blockname]-layout"
 		// so we need to normalize the random id.
-		$normalized_html = preg_replace( '/wp-container-\d+/', 'wp-container-1', $html );
+		$normalized_html = preg_replace( '/wp-container-[a-z-]+\d+/', 'wp-container-1', $html );
 
 		// The gallery block uses a unique class name of `wp_unique_id( 'wp-block-gallery-' )`
 		// so we need to normalize the random id.

tests/phpunit/tests/functions/wpUniquePrefixedId.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * Test cases for the `wp_unique_prefixed_id()` function.
+ *
+ * @package WordPress\UnitTests
+ *
+ * @since 6.4
+ *
+ * @group functions.php
+ * @covers ::wp_unique_prefixed_id
+ */
+class Tests_Functions_WpUniquePrefixedId extends WP_UnitTestCase {
+
+	public function test_wp_unique_prefixed_id() {
+		$first          = wp_unique_prefixed_id( 'test' );
+		$second         = wp_unique_prefixed_id( 'test' );
+		$null_id        = wp_unique_prefixed_id( null );
+		$second_null_id = wp_unique_prefixed_id( null );
+		$default        = wp_unique_prefixed_id();
+		$second_default = wp_unique_prefixed_id();
+		$this->assertNotEquals( $first, $second );
+		$this->assertNotEquals( $default, $second_default );
+		$this->assertNotEquals( $null_id, $second_null_id );
+	}
+}