Hide expander from accessibility tree and add comment

packages/block-editor/src/components/block-navigation/expander.js

@@ -5,10 +5,18 @@ import { chevronRightSmall, Icon } from '@wordpress/icons';
 export default function BlockNavigationExpander( { onClick } ) {
 	return (
 		// Keyboard events are handled by TreeGrid see: components/src/tree-grid/index.js
+		//
+		// The expander component is implemented as a pseudo element in the w3 example
+		// https://www.w3.org/TR/wai-aria-practices/examples/treegrid/treegrid-1.html
+		//
+		// We've mimicked this by adding an icon with aria-hidden set to true to hide this from the accessibility tree.
+		// For the current tree grid implementation, please do not try to make this a button.
+		//
 		// eslint-disable-next-line jsx-a11y/click-events-have-key-events,jsx-a11y/no-static-element-interactions
 		<span
 			className="block-editor-block-navigation__expander"
 			onClick={ ( event ) => onClick( event, { forceToggle: true } ) }
+			aria-hidden="true"
 		>
 			<Icon icon={ chevronRightSmall } />
 		</span>