Mark active item in table of contents (#733)

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");
+  }
+}