From 728620b527dd4d8e308c9344e319e5e11bdad859 Mon Sep 17 00:00:00 2001
From: Philipp Oppermann <dev@phil-opp.com>
Date: Fri, 31 Jan 2020 14:17:22 +0100
Subject: [PATCH] Mark active item in table of contents (#733)

---
 blog/static/css/main.css | 11 +++++++++--
 blog/static/js/main.js   | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/blog/static/css/main.css b/blog/static/css/main.css
index b0f18a5c..7f940cbb 100644
--- a/blog/static/css/main.css
+++ b/blog/static/css/main.css
@@ -193,14 +193,21 @@ aside#all-posts-link {
     margin-left: -15rem;
     font-size: 90%;
     line-height: 1.2;
+  }
+
+  #toc-aside li:not(.active) > a, #toc-aside h2 {
     opacity: .5;
     transition: opacity .5s;
   }
 
-  #toc-aside:hover {
+  #toc-aside:hover li:not(.active) > a, #toc-aside:hover h2 {
     opacity: 1;
   }
 
+  #toc-aside li.active > a {
+    font-weight: bold;
+  }
+
   #toc-aside h2 {
     font-size: 110%;
     margin-bottom: .2rem;
@@ -212,7 +219,7 @@ aside#all-posts-link {
     list-style:none;
   }
 
-  #toc-aside ol li:before {
+  #toc-aside ol li a:before {
     content: "";
     border-color: transparent #008eef;
     border-style: solid;
diff --git a/blog/static/js/main.js b/blog/static/js/main.js
index 96ed6181..ce6b080b 100644
--- a/blog/static/js/main.js
+++ b/blog/static/js/main.js
@@ -3,6 +3,8 @@ window.onload = function() {
 
   if (container != null) {
     resize_toc(container);
+    toc_scroll_position(container);
+    window.onscroll = function() { toc_scroll_position(container) };
   }
 }
 
@@ -24,3 +26,38 @@ function resize_toc(container) {
     resizeId = setTimeout(resize, 300);
   };
 }
+
+function toc_scroll_position(container) {
+  if (container.offsetParent === null) {
+    // skip computation if ToC is not visible
+    return;
+  }
+  var items = container.querySelectorAll("li")
+
+  // remove active class for all items
+  for (item of container.querySelectorAll("li")) {
+    item.classList.remove("active");
+  }
+
+  // look for active item
+  var site_offset = document.documentElement.scrollTop;
+  var current_toc_item = null;
+  for (item of container.querySelectorAll("li")) {
+    if (item.offsetParent === null) {
+      // skip items that are not visible
+      continue;
+    }
+    var anchor = item.firstElementChild.getAttribute("href");
+    var heading = document.querySelector(anchor);
+    if (heading.offsetTop <= (site_offset + document.documentElement.clientHeight / 3)) {
+      current_toc_item = item;
+    } else {
+      break;
+    }
+  }
+
+  // set active call for current ToC item
+  if (current_toc_item != null) {
+    current_toc_item.classList.add("active");
+  }
+}