(editor)(fix) Append the edit-post-header-toolbar class in Navigabl…

…eToolbar for backward compatibility with plugin GUI injections (#58154)

* Keep the `edit-post-header-toolbar` CSS class in the `NavigableToolbar` as some plugins use it to append GUIs to it

* Add comment justifying the change

packages/editor/src/components/document-tools/index.js

@@ -104,8 +104,16 @@ function DocumentTools( {
 	const shortLabel = ! isInserterOpened ? __( 'Add' ) : __( 'Close' );
 
 	return (
+		// Some plugins expect and use the `edit-post-header-toolbar` CSS class to
+		// find the toolbar and inject UI elements into it. This is not officially
+		// supported, but we're keeping it in the list of class names for backwards
+		// compatibility.
 		<NavigableToolbar
-			className={ classnames( 'editor-document-tools', className ) }
+			className={ classnames(
+				'editor-document-tools',
+				'edit-post-header-toolbar',
+				className
+			) }
 			aria-label={ toolbarAriaLabel }
 			shouldUseKeyboardFocusShortcut={ ! blockToolbarCanBeFocused }
 			variant="unstyled"