Post Template: Output number of columns as classname for backwards co…

…mpatibility (#51358)

* Grid Layout: Output number of columns as classname for backwards compatibility

* Refactor to only apply to the post template block

packages/block-library/src/post-template/edit.js

@@ -222,7 +222,10 @@ export default function PostTemplateEdit( {
 	);
 
 	const blockProps = useBlockProps( {
-		className: classnames( __unstableLayoutClassNames ),
+		className: classnames( __unstableLayoutClassNames, {
+			[ `columns-${ columnCount }` ]:
+				layoutType === 'grid' && columnCount, // Ensure column count is flagged via classname for backwards compatibility.
+		} ),
 	} );
 
 	if ( ! posts ) {

packages/block-library/src/post-template/index.php

@@ -72,6 +72,11 @@ function render_block_core_post_template( $attributes, $content, $block ) {
 		$classnames .= ' has-link-color';
 	}
 
+	// Ensure backwards compatibility by flagging the number of columns via classname when using grid layout.
+	if ( isset( $attributes['layout']['type'] ) && 'grid' === $attributes['layout']['type'] && ! empty( $attributes['layout']['columnCount'] ) ) {
+		$classnames .= ' ' . sanitize_title( 'columns-' . $attributes['layout']['columnCount'] );
+	}
+
 	$wrapper_attributes = get_block_wrapper_attributes( array( 'class' => trim( $classnames ) ) );
 
 	$content = '';