Writing flow: split heading into default block (#61891)

Unlinked contributors: erikiva.

Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>

packages/block-editor/src/store/actions.js

@@ -982,7 +982,7 @@ export const __unstableSplitSelection =
 			},
 		};
 
-		const tail = {
+		let tail = {
 			...blockB,
 			// Only preserve the original client ID if the end is different.
 			clientId:
@@ -995,6 +995,26 @@ export const __unstableSplitSelection =
 			},
 		};
 
+		// When splitting a block, attempt to convert the tail block to the
+		// default block type. For example, when splitting a heading block, the
+		// tail block will be converted to a paragraph block. Note that for
+		// blocks such as a list item and button, this will be skipped because
+		// the default block type cannot be inserted.
+		const defaultBlockName = getDefaultBlockName();
+		if (
+			// A block is only split when the selection is within the same
+			// block.
+			blockA.clientId === blockB.clientId &&
+			defaultBlockName &&
+			tail.name !== defaultBlockName &&
+			select.canInsertBlockType( defaultBlockName, anchorRootClientId )
+		) {
+			const switched = switchToBlockType( tail, defaultBlockName );
+			if ( switched?.length === 1 ) {
+				tail = switched[ 0 ];
+			}
+		}
+
 		if ( ! blocks.length ) {
 			dispatch.replaceBlocks( select.getSelectedBlockClientIds(), [
 				head,

test/e2e/specs/editor/various/copy-cut-paste.spec.js

@@ -695,10 +695,9 @@ test.describe( 'Copy/cut/paste', () => {
 				},
 			},
 			{
-				name: 'core/heading',
+				name: 'core/paragraph',
 				attributes: {
 					content: 'bB',
-					level: 2,
 				},
 			},
 		] );