Add polyfill and comments to utils/sticky

app/assets/javascripts/init_changes_dropdown.js

 import $ from 'jquery';
-import stickyMonitor from './lib/utils/sticky';
+import { stickyMonitor } from './lib/utils/sticky';
 
 export default (stickyTop) => {
   stickyMonitor(document.querySelector('.js-diff-files-changed'), stickyTop);

app/assets/javascripts/job.js

 import $ from 'jquery';
 import _ from 'underscore';
-import StickyFill from 'stickyfilljs';
+import { polyfillSticky } from './lib/utils/sticky';
 import axios from './lib/utils/axios_utils';
 import { visitUrl } from './lib/utils/url_utility';
 import bp from './breakpoints';
@@ -80,14 +80,7 @@ export default class Job {
   }
 
   initAffixTopArea() {
-    /**
-      If the browser does not support position sticky, it returns the position as static.
-      If the browser does support sticky, then we allow the browser to handle it, if not
-      then we use a polyfill
-    */
-    if (this.$topBar.css('position') !== 'static') return;
-
-    StickyFill.add(this.$topBar);
+    polyfillSticky(this.$topBar);
   }
 
   // eslint-disable-next-line class-methods-use-this

app/assets/javascripts/lib/utils/sticky.js

+import StickyFill from 'stickyfilljs';
+
 export const createPlaceholder = () => {
   const placeholder = document.createElement('div');
   placeholder.classList.add('sticky-placeholder');
@@ -28,7 +30,16 @@ export const isSticky = (el, scrollY, stickyTop, insertPlaceholder) => {
   }
 };
 
-export default (el, stickyTop, insertPlaceholder = true) => {
+/**
+ * Create a listener that will toggle a 'is-stuck' class, based on the current scroll position.
+ *
+ * - If the current environment does not support `position: sticky`, do nothing.
+ *
+ * @param {HTMLElement} el The `position: sticky` element.
+ * @param {Number} stickyTop Used to determine when an element is stuck.
+ * @param {Boolean} insertPlaceholder Should a placeholder element be created when element is stuck?
+ */
+export const stickyMonitor = (el, stickyTop, insertPlaceholder = true) => {
   if (!el) return;
 
   if (typeof CSS === 'undefined' || !(CSS.supports('(position: -webkit-sticky) or (position: sticky)'))) return;
@@ -37,3 +48,13 @@ export default (el, stickyTop, insertPlaceholder = true) => {
     passive: true,
   });
 };
+
+/**
+ * Polyfill the `position: sticky` behavior.
+ *
+ * - If the current environment supports `position: sticky`, do nothing.
+ * - Can receive an iterable element list (NodeList, jQuery collection, etc.) or single HTMLElement.
+ */
+export const polyfillSticky = (el) => {
+  StickyFill.add(el);
+};