Stop Expansion of Top Level Menu in the WordPress Admin Sidebar

You can use this code

add_action('adminmenu', function() {
   ?>
    <script>
    jQuery('#adminmenu .wp-has-current-submenu.wp-menu-open')
      .removeClass('wp-has-current-submenu')
      .removeClass('wp-menu-open')
      .addClass('wp-not-current-submenu');
    </script>
    <?php
});