Update: Keep additional classes during a block transform. (WordPress#38964)

Author: jorgefilipecosta

packages/block-editor/src/hooks/custom-class-name.js

@@ -112,6 +112,45 @@ export function addSaveProps( extraProps, blockType, attributes ) {
 	return extraProps;
 }
 
+export function addTransforms( result, source, index, results ) {
+	if ( ! hasBlockSupport( result.name, 'customClassName', true ) ) {
+		return result;
+	}
+
+	// If the condition verifies we are probably in the presence of a wrapping transform
+	// e.g: nesting paragraphs in a group or columns and in that case the class should not be kept.
+	if ( results.length === 1 && result.innerBlocks.length === source.length ) {
+		return result;
+	}
+
+	// If we are transforming one block to multiple blocks or multiple blocks to one block,
+	// we ignore the class during the transform.
+	if (
+		( results.length === 1 && source.length > 1 ) ||
+		( results.length > 1 && source.length === 1 )
+	) {
+		return result;
+	}
+
+	// If we are in presence of transform between one or more block in the source
+	// that have one or more blocks in the result
+	// we apply the class on source N to the result N,
+	// if source N does not exists we do nothing.
+	if ( source[ index ] ) {
+		const originClassName = source[ index ]?.attributes.className;
+		if ( originClassName ) {
+			return {
+				...result,
+				attributes: {
+					...result.attributes,
+					className: originClassName,
+				},
+			};
+		}
+	}
+	return result;
+}
+
 addFilter(
 	'blocks.registerBlockType',
 	'core/custom-class-name/attribute',
@@ -127,3 +166,9 @@ addFilter(
 	'core/custom-class-name/save-props',
 	addSaveProps
 );
+
+addFilter(
+	'blocks.switchToBlockType.transformedBlock',
+	'core/color/addTransforms',
+	addTransforms
+);