first commit

media/templates/site/cassiopeia/js/mod_menu/menu-metismenu.js

@@ -0,0 +1,24 @@
+/**
+ * @package     Joomla.Site
+ * @subpackage  Templates.cassiopeia
+ * @copyright   (C) 2020 Open Source Matters, Inc. <https://www.joomla.org>
+ * @license     GNU General Public License version 2 or later; see LICENSE.txt
+ * @since       4.0.0
+ */
+
+document.addEventListener('DOMContentLoaded', () => {
+  const allMenus = document.querySelectorAll('ul.mod-menu_dropdown-metismenu');
+  allMenus.forEach(menu => {
+    // eslint-disable-next-line no-new, no-undef
+    const mm = new MetisMenu(menu, {
+      triggerElement: 'button.mm-toggler'
+    }).on('shown.metisMenu', event => {
+      window.addEventListener('click', function mmClick(e) {
+        if (!event.target.contains(e.target)) {
+          mm.hide(event.detail.shownElement);
+          window.removeEventListener('click', mmClick);
+        }
+      });
+    });
+  });
+});

media/templates/site/cassiopeia/js/mod_menu/menu-metismenu.min.js

@@ -0,0 +1 @@
+document.addEventListener("DOMContentLoaded",(()=>{document.querySelectorAll("ul.mod-menu_dropdown-metismenu").forEach((e=>{const n=new MetisMenu(e,{triggerElement:"button.mm-toggler"}).on("shown.metisMenu",(e=>{window.addEventListener("click",(function t(o){e.target.contains(o.target)||(n.hide(e.detail.shownElement),window.removeEventListener("click",t))}))}))}))}));