Gallery block: turn on auto-migration of v1 Gallery blocks to v2 format when edited

lib/compat.php

@@ -191,3 +191,54 @@ function gutenberg_safe_style_attrs( $attrs ) {
 	return $attrs;
 }
 add_filter( 'safe_style_css', 'gutenberg_safe_style_attrs' );
+
+/**
+ * The new gallery block format is not compatible with the use_BalanceTags option
+ * in WP versions <= 5.8 https://core.trac.wordpress.org/ticket/54130.
+ * This method adds a variable to the wp namespace to indicate if the new gallery block
+ * format can be enabled or not. It needs to be added this early and to the wp namespace
+ * as it needs to be available when the intial block parsing runs on editor load, and most of
+ * the editor store and standard flags are not loaded yet at that point
+ *
+ * @since 12.1.0
+ * @todo This should be removed when the minimum required WP version is >= 5.9.
+ *
+ * @return void.
+ */
+function gutenberg_check_gallery_block_v2_compatibility() {
+	$use_balance_tags   = (int) get_option( 'use_balanceTags' );
+	$v2_gallery_enabled = boolval( 1 !== $use_balance_tags || is_wp_version_compatible( '5.9' ) ) ? 'true' : 'false';
+
+	wp_add_inline_script(
+		'wp-dom-ready',
+		'wp.galleryBlockV2Enabled = ' . $v2_gallery_enabled . ';',
+		'after'
+	);
+}
+add_action( 'init', 'gutenberg_check_gallery_block_v2_compatibility' );
+
+/**
+ * Prevent use_balanceTags being enabled on WordPress 5.8 or earlier as it breaks
+ * the layout of the new Gallery block.
+ *
+ * @since 12.1.0
+ * @todo This should be removed when the minimum required WP version is >= 5.9.
+ *
+ * @param int $new_value The new value for use_balanceTags.
+ */
+function gutenberg_use_balancetags_check( $new_value ) {
+	global $wp_version;
+
+	if ( 1 === (int) $new_value && version_compare( $wp_version, '5.9', '<' ) ) {
+		/* translators: %s: Minimum required version */
+		$message = sprintf( __( 'Gutenberg requires WordPress %s or later in order to enable the &#8220;Correct invalidly nested XHTML automatically&#8221; option. Please upgrade WordPress before enabling.', 'gutenberg' ), '5.9' );
+		add_settings_error( 'gutenberg_use_balancetags_check', 'gutenberg_use_balancetags_check', $message, 'error' );
+		if ( class_exists( 'WP_CLI' ) ) {
+			WP_CLI::error( $message );
+		}
+		return 0;
+	}
+
+	return $new_value;
+}
+add_filter( 'pre_update_option_use_balanceTags', 'gutenberg_use_balancetags_check' );

lib/experiments-page.php

@@ -101,7 +101,7 @@ function gutenberg_experiments_editor_settings( $settings ) {
 	// This bypass needs to remain in place until this is resolved and a patch released.
 	// https://core.trac.wordpress.org/ticket/54130.
 	$experiments_settings = array(
-		'__unstableGalleryWithImageBlocks' => (int) get_option( 'use_balanceTags' ) !== 1,
+		'__unstableGalleryWithImageBlocks' => (int) get_option( 'use_balanceTags' ) !== 1 || is_wp_version_compatible( '5.9' ),
 	);
 	return array_merge( $settings, $experiments_settings );
 }