How to disable admin flyout menus?

It’s a bit of a workarround but here you go:

function remove_flyout() {
   echo '<style type="text/css">
        .js #adminmenu .wp-submenu.sub-open {
            display: none;
        }
    </style>';
}
add_action('admin_head', 'remove_flyout');

Put it in your functions.php