navbar: center active menu item on mobile

js/navbar.js

+/* Copyright (C) 2018 Pietro Albini <pietroalbini@ubuntu.com>
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU Affero General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+function ubuntuit_navbar_center_active() {
+    var navs = document.querySelectorAll("nav");
+    for (var i = 0; i < navs.length; i++) {
+        var active = navs[i].querySelector(".active");
+        if (active !== undefined) {
+            var offset = active.getBoundingClientRect().left;
+            navs[i].scrollBy(offset - window.innerWidth / 2 + active.offsetWidth / 2, 0);
+        }
+    }
+};
+
+window.addEventListener("load", ubuntuit_navbar_center_active());