From 05c64bde9b5081a1cf0733df0bba70aa077b4f44 Mon Sep 17 00:00:00 2001
From: lfalin <lee.falin@gmail.com>
Date: Sat, 27 May 2017 18:27:37 -0600
Subject: [PATCH] Fix anchor scrolling that hides behind top nav bar

---
 static/js/learn.js | 88 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/static/js/learn.js b/static/js/learn.js
index b776ac7ba53..856c3320b8b 100644
--- a/static/js/learn.js
+++ b/static/js/learn.js
@@ -225,6 +225,94 @@ jQuery(document).ready(function() {
     $('.progress').hover(function() {
         $('.progress').stop(true, false, true).fadeToggle(100);
     });
+
+    /** 
+    * Fix anchor scrolling that hides behind top nav bar
+    * Courtesy of https://stackoverflow.com/a/13067009/28106
+    *
+    * We could use pure css for this if only heading anchors were
+    * involved, but this works for any anchor, including footnotes
+    **/
+    (function(document, history, location) {
+      var HISTORY_SUPPORT = !!(history && history.pushState);
+
+      var anchorScrolls = {
+        ANCHOR_REGEX: /^#[^ ]+$/,
+        OFFSET_HEIGHT_PX: 50,
+
+        /**
+         * Establish events, and fix initial scroll position if a hash is provided.
+         */
+        init: function() {
+          this.scrollToCurrent();
+          window.addEventListener('hashchange', this.scrollToCurrent.bind(this));
+          document.body.addEventListener('click', this.delegateAnchors.bind(this));
+        },
+
+        /**
+         * Return the offset amount to deduct from the normal scroll position.
+         * Modify as appropriate to allow for dynamic calculations
+         */
+        getFixedOffset: function() {
+          return this.OFFSET_HEIGHT_PX;
+        },
+
+        /**
+         * If the provided href is an anchor which resolves to an element on the
+         * page, scroll to it.
+         * @param  {String} href
+         * @return {Boolean} - Was the href an anchor.
+         */
+        scrollIfAnchor: function(href, pushToHistory) {
+          var match, rect, anchorOffset;
+
+          if(!this.ANCHOR_REGEX.test(href)) {
+            return false;
+          }
+
+          match = document.getElementById(href.slice(1));
+
+          if(match) {
+            rect = match.getBoundingClientRect();
+            anchorOffset = window.pageYOffset + rect.top - this.getFixedOffset();
+            window.scrollTo(window.pageXOffset, anchorOffset);
+
+            // Add the state to history as-per normal anchor links
+            if(HISTORY_SUPPORT && pushToHistory) {
+              history.pushState({}, document.title, location.pathname + href);
+            }
+          }
+
+          return !!match;
+        },
+
+        /**
+         * Attempt to scroll to the current location's hash.
+         */
+        scrollToCurrent: function() {
+          this.scrollIfAnchor(window.location.hash);
+        },
+
+        /**
+         * If the click event's target was an anchor, fix the scroll position.
+         */
+        delegateAnchors: function(e) {
+          var elem = e.target;
+
+          if(
+            elem.nodeName === 'A' &&
+            this.scrollIfAnchor(elem.getAttribute('href'), true)
+          ) {
+            e.preventDefault();
+          }
+        }
+      };
+
+      window.addEventListener(
+        'DOMContentLoaded', anchorScrolls.init.bind(anchorScrolls)
+      );
+    })(window.document, window.history, window.location);
+
 });
 
 jQuery(window).on('load', function() {