Classic Block: fix undo keyboard shortcut (#23376)

* Prevent gutenberg undo from kicking in when in classic block
Co-authored-by: Glen Davies <glen.davies@a8c.com>

packages/block-library/src/classic/edit.js

@@ -8,7 +8,7 @@ import { debounce } from 'lodash';
  */
 import { Component } from '@wordpress/element';
 import { __, _x } from '@wordpress/i18n';
-import { BACKSPACE, DELETE, F10 } from '@wordpress/keycodes';
+import { BACKSPACE, DELETE, F10, isKeyboardEvent } from '@wordpress/keycodes';
 
 const { wp } = window;
 
@@ -136,6 +136,11 @@ export default class ClassicEdit extends Component {
 		);
 
 		editor.on( 'keydown', ( event ) => {
+			if ( isKeyboardEvent.primary( event, 'z' ) ) {
+				// Prevent the gutenberg undo kicking in so TinyMCE undo stack works as expected
+				event.stopPropagation();
+			}
+
 			if (
 				( event.keyCode === BACKSPACE || event.keyCode === DELETE ) &&
 				isTmceEmpty( editor )