moved JS & only run when new_nav cookie exists

app/assets/javascripts/fly_out_nav.js

+export const calculateTop = (boundingRect, outerHeight) => {
+  const windowHeight = window.innerHeight;
+  const bottomOverflow = windowHeight - (boundingRect.top + outerHeight);
+
+  return bottomOverflow < 0 ? boundingRect.top - Math.abs(bottomOverflow) : boundingRect.top;
+};
+
+export const createArrowStyles = (boundingRect, top) => `.sidebar-sub-level-items::before { transform: translate3d(0, ${boundingRect.top - top}px, 0); }`;
+
+export default () => {
+  const style = document.createElement('style');
+  document.head.appendChild(style);
+
+  $('.sidebar-top-level-items > li:not(.active)').on('mouseover', (e) => {
+    const $this = e.currentTarget;
+    const $subitems = $('.sidebar-sub-level-items', $this).show();
+
+    if ($subitems.length) {
+      const boundingRect = $this.getBoundingClientRect();
+      const top = calculateTop(boundingRect, $subitems.outerHeight());
+
+      $subitems.css({
+        transform: `translate3d(0, ${top}px, 0)`,
+      });
+
+      style.sheet.insertRule(createArrowStyles(boundingRect, top), 0);
+    }
+  }).on('mouseout', (e) => {
+    $('.sidebar-sub-level-items', e.currentTarget).hide();
+
+    if (style.sheet.rules.length) {
+      style.sheet.deleteRule(0);
+    }
+  });
+};

app/assets/javascripts/layout_nav.js

 /* eslint-disable func-names, space-before-function-paren, no-var, prefer-arrow-callback, no-unused-vars, one-var, one-var-declaration-per-line, vars-on-top, max-len */
+import Cookies from 'js-cookie';
 import _ from 'underscore';
+import initFlyOutNav from './fly_out_nav';
 
 (function() {
   var hideEndFade;
@@ -54,5 +56,9 @@ import _ from 'underscore';
 
   $(() => {
     $(window).on('scroll', _.throttle(applyScrollNavClass, 100));
+
+    if (Cookies.get('new_nav') === 'true') {
+      initFlyOutNav();
+    }
   });
 }).call(window);

app/assets/javascripts/main.js

@@ -347,31 +347,4 @@ $(function () {
   gl.utils.renderTimeago();
 
   $(document).trigger('init.scrolling-tabs');
-
-  const style = document.createElement("style");
-  document.head.appendChild(style);
-
-  $('.sidebar-top-level-items > li:not(.active)').on('mouseover', (e) => {
-    const windowHeight = window.innerHeight;
-    const $this = e.currentTarget;
-    const $subitems = $('.sidebar-sub-level-items', $this).show();
-
-    if ($subitems.length) {
-      const boundingRect = $this.getBoundingClientRect();
-      const bottomOverflow = windowHeight - (boundingRect.top + $subitems.outerHeight());
-      const top = bottomOverflow < 0 ? boundingRect.top - Math.abs(bottomOverflow) : boundingRect.top;
-
-      $subitems.css({
-        transform: `translate3d(0, ${top}px, 0)`,
-      });
-
-      style.sheet.insertRule(`.sidebar-sub-level-items::before { transform: translate3d(0, ${boundingRect.top - top}px, 0); }`, 0);
-    }
-  }).on('mouseout', (e) => {
-    $('.sidebar-sub-level-items', e.currentTarget).hide();
-
-    if (style.sheet.rules.length) {
-      style.sheet.deleteRule(0);
-    }
-  });
 });

spec/javascripts/fly_out_nav_spec.js

+import { calculateTop, createArrowStyles } from '~/fly_out_nav';
+
+describe('Fly out sidebar navigation', () => {
+  describe('calculateTop', () => {
+    it('returns boundingRect top', () => {
+      const boundingRect = {
+        top: 100,
+      };
+
+      expect(
+        calculateTop(boundingRect, 100),
+      ).toBe(100);
+    });
+
+    it('returns boundingRect - bottomOverflow', () => {
+      const boundingRect = {
+        top: window.innerHeight,
+      };
+
+      expect(
+        calculateTop(boundingRect, 100),
+      ).toBe(window.innerHeight - 100);
+    });
+  });
+
+  describe('createArrowStyles', () => {
+    it('returns translate3d styles', () => {
+      const boundingRect = {
+        top: 100,
+      };
+
+      expect(
+        createArrowStyles(boundingRect, 50),
+      ).toContain('translate3d(0, 50px, 0)');
+    });
+  });
+});