Post title: fix single line paste (#42321)

packages/editor/src/components/post-title/index.js

@@ -149,9 +149,13 @@ function PostTitle( _, forwardedRef ) {
 			plainText,
 		} );
 
-		if ( typeof content !== 'string' && content.length ) {
-			event.preventDefault();
+		event.preventDefault();
+
+		if ( ! content.length ) {
+			return;
+		}
 
+		if ( typeof content !== 'string' ) {
 			const [ firstBlock ] = content;
 
 			if (
@@ -164,6 +168,8 @@ function PostTitle( _, forwardedRef ) {
 			} else {
 				onInsertBlockAfter( content );
 			}
+		} else {
+			onUpdate( content );
 		}
 	}
 

test/e2e/specs/editor/various/__snapshots__/Copy-cut-paste-should-paste-single-line-in-post-title-1-chromium.txt

@@ -0,0 +1 @@
+Hello World

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

@@ -397,4 +397,23 @@ test.describe( 'Copy/cut/paste', () => {
 		await pageUtils.pressKeyWithModifier( 'primary', 'v' );
 		expect( await editor.getEditedPostContent() ).toMatchSnapshot();
 	} );
+
+	test( 'should paste single line in post title', async ( {
+		page,
+		pageUtils,
+	} ) => {
+		// This test checks whether we are correctly handling single line
+		// pasting in the post title. Previously we were accidentally falling
+		// back to default browser behaviour, allowing the browser to insert
+		// unfiltered HTML. When we swap out the post title in the post editor
+		// with the proper block, this test can be removed.
+		await pageUtils.setClipboardData( {
+			html: '<span style="border: 1px solid black">Hello World</span>',
+		} );
+		await pageUtils.pressKeyWithModifier( 'primary', 'v' );
+		// Expect the span to be filtered out.
+		expect(
+			await page.evaluate( () => document.activeElement.innerHTML )
+		).toMatchSnapshot();
+	} );
 } );