From bdfbb504b75fc6478a7031cdfe8a00c372b43e30 Mon Sep 17 00:00:00 2001
From: Marin Atanasov <8436925+tyxla@users.noreply.github.com>
Date: Tue, 14 Jul 2026 12:01:03 +0300
Subject: [PATCH] Block Supports: Improve handling of block class name to avoid
 fatal

---
 .../block-supports/block-style-variations.php |  7 ++-
 src/wp-includes/block-supports/layout.php     |  4 +-
 .../block-supports/block-style-variations.php | 43 +++++++++++++++++++
 tests/phpunit/tests/block-supports/layout.php | 25 +++++++++++
 4 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/src/wp-includes/block-supports/block-style-variations.php b/src/wp-includes/block-supports/block-style-variations.php
index f508003417f2c..5daccd629c055 100644
--- a/src/wp-includes/block-supports/block-style-variations.php
+++ b/src/wp-includes/block-supports/block-style-variations.php
@@ -223,11 +223,16 @@ function wp_render_block_style_variation_class_name( $block_content, $block ) {
 		return $block_content;
 	}
 
+	$block_class_name = $block['attrs']['className'];
+	if ( ! is_string( $block_class_name ) ) {
+		return $block_content;
+	}
+
 	/*
 	 * Matches a class prefixed by `is-style`, followed by the
 	 * variation slug, then `--`, and finally an instance number.
 	 */
-	preg_match( '/\bis-style-(\S+?--\d+)\b/', $block['attrs']['className'], $matches );
+	preg_match( '/\bis-style-(\S+?--\d+)\b/', $block_class_name, $matches );
 
 	if ( empty( $matches ) ) {
 		return $block_content;
diff --git a/src/wp-includes/block-supports/layout.php b/src/wp-includes/block-supports/layout.php
index 158ae68fb3c59..105f4e28ecc1c 100644
--- a/src/wp-includes/block-supports/layout.php
+++ b/src/wp-includes/block-supports/layout.php
@@ -1160,7 +1160,9 @@ function wp_render_layout_support_flag( $block_content, $block ) {
 		// Check if the block has an active style variation with a blockGap value.
 		// Only check the registry if the className contains a variation class to avoid unnecessary lookups.
 		$variation_block_gap_value = null;
-		$block_class_name          = $block['attrs']['className'] ?? '';
+		$block_class_name          = is_string( $block['attrs']['className'] ?? null )
+			? $block['attrs']['className']
+			: '';
 		if ( $block_class_name && str_contains( $block_class_name, 'is-style-' ) && $block_name ) {
 			$styles_registry   = WP_Block_Styles_Registry::get_instance();
 			$registered_styles = $styles_registry->get_registered_styles_for_block( $block_name );
diff --git a/tests/phpunit/tests/block-supports/block-style-variations.php b/tests/phpunit/tests/block-supports/block-style-variations.php
index 0c56d0531829b..0627820bd8d4d 100644
--- a/tests/phpunit/tests/block-supports/block-style-variations.php
+++ b/tests/phpunit/tests/block-supports/block-style-variations.php
@@ -320,4 +320,47 @@ public function test_block_style_variation_ref_values() {
 
 		$this->assertSameSetsWithIndex( $expected, $variation_data, 'Variation data with resolved ref values does not match' );
 	}
+
+	/**
+	 * Tests that a non-string `className` attribute does not cause a fatal
+	 * error and the block content is returned unmodified.
+	 *
+	 * @covers ::wp_render_block_style_variation_class_name
+	 */
+	public function test_block_style_variation_class_name_with_non_string_class_name() {
+		$block = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(
+				'className' => array( '0', '1' ),
+			),
+		);
+
+		$block_content = "<p class=\"0 1\">Test</p>\n";
+
+		$this->assertSame(
+			$block_content,
+			wp_render_block_style_variation_class_name( $block_content, $block ),
+			'Block content should be returned unchanged when className is not a string'
+		);
+	}
+
+	/**
+	 * Tests to ensure that there are no references to an undefined array key
+	 * if `className` is not assigned.
+	 *
+	 * @covers ::wp_render_block_style_variation_class_name
+	 */
+	public function test_block_style_variation_class_name_with_missing_class_name() {
+		$block = array(
+			'blockName' => 'core/paragraph',
+			'attrs'     => array(),
+		);
+
+		$block_content = "<p>Test</p>\n";
+
+		$this->assertSame(
+			$block_content,
+			wp_render_block_style_variation_class_name( $block_content, $block )
+		);
+	}
 }
diff --git a/tests/phpunit/tests/block-supports/layout.php b/tests/phpunit/tests/block-supports/layout.php
index c52c0aca656a1..95a8577730e21 100644
--- a/tests/phpunit/tests/block-supports/layout.php
+++ b/tests/phpunit/tests/block-supports/layout.php
@@ -1053,4 +1053,29 @@ public function data_get_block_style_variation_name_from_registered_style() {
 			),
 		);
 	}
+
+	/**
+	 * Tests that a non-string `className` attribute does not cause a fatal
+	 * when checking for style variation layout styles.
+	 *
+	 * @covers ::wp_render_layout_support_flag
+	 */
+	public function test_layout_support_flag_with_non_string_class_name() {
+		$block_content = '<div class="wp-block-group 0 1"></div>';
+		$block         = array(
+			'blockName' => 'core/group',
+			'attrs'     => array(
+				'className' => array( '0', '1' ),
+				'layout'    => array(
+					'type' => 'constrained',
+				),
+			),
+		);
+
+		$this->assertSame(
+			'<div class="wp-block-group 0 1 is-layout-constrained wp-block-group-is-layout-constrained"></div>',
+			wp_render_layout_support_flag( $block_content, $block ),
+			'Layout support should render the expected markup when className is not a string'
+		);
+	}
 }