fix: navigation in dropdown menu with a separator (#9474)

Author: microbit-robert

core/menu.ts

@@ -274,7 +274,7 @@ export class Menu {
    */
   highlightNext() {
     const index = this.highlightedItem
-      ? this.menuItems.indexOf(this.highlightedItem)
+      ? this.getMenuItems().indexOf(this.highlightedItem)
       : -1;
     this.highlightHelper(index, 1);
   }
@@ -287,7 +287,7 @@ export class Menu {
    */
   highlightPrevious() {
     const index = this.highlightedItem
-      ? this.menuItems.indexOf(this.highlightedItem)
+      ? this.getMenuItems().indexOf(this.highlightedItem)
       : -1;
     this.highlightHelper(index < 0 ? this.menuItems.length : index, -1);
   }