ArtfulPussycat

/** Fixed Mobile Menu **/
@media (max-width: 980px) {
.et_non_fixed_nav.et_transparent_nav #main-header, .et_non_fixed_nav.et_transparent_nav #top-header, .et_fixed_nav #main-header, .et_fixed_nav #top-header {
    position: fixed !important; } }
.et_mobile_menu {
    overflow: scroll !important;
    max-height: 83vh;
}

ArtfulPussycat

<?php
/**
 * Plugin Name:       Your Functionality MU-Plugin Name
 * Plugin URI:        http://example.com/plugin-name-uri/
 * Description:       This is a short description of what the plugin does. It's displayed in the WordPress admin area.
 * Version:           1.0.0
 * Author:            Your Name or Your Company
 * Author URI:        http://example.com/
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt