Expose the editable root host as a labelled textbox

While the writing flow wrapper is the editing host it takes focus, but
carried no role or accessible name, so assistive technology announced an
unlabelled editable region. Mark it as a multiline textbox named "Editor
canvas" while it is the host, and remove the attributes when it stops.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ellatrix

packages/block-editor/src/components/writing-flow/use-editable-root.js

@@ -1,6 +1,7 @@
 /**
  * WordPress dependencies
  */
+import { __ } from '@wordpress/i18n';
 import { useSelect } from '@wordpress/data';
 import { useRefEffect } from '@wordpress/compose';
 import { hasBlockSupport } from '@wordpress/blocks';
@@ -76,6 +77,13 @@ export default function useEditableRoot() {
 					}
 				}
 
+				// Expose the host as a named multiline textbox so it has a
+				// role and accessible name once it takes focus. The label is
+				// generic because the host can span several blocks.
+				node.setAttribute( 'role', 'textbox' );
+				node.setAttribute( 'aria-multiline', 'true' );
+				node.setAttribute( 'aria-label', __( 'Editor canvas' ) );
+
 				// Move focus from the block's editable element to the
 				// wrapper, but only when an editable element belonging to
 				// the selected block has focus. Never steal focus from other
@@ -110,6 +118,9 @@ export default function useEditableRoot() {
 				! isMultiSelecting()
 			) {
 				node.setAttribute( 'contenteditable', 'false' );
+				node.removeAttribute( 'role' );
+				node.removeAttribute( 'aria-multiline' );
+				node.removeAttribute( 'aria-label' );
 
 				// If the wrapper held focus, return focus to the editable
 				// element containing the selection, which is focusable again