Squashed 'themes/codex/' content from commit da2d16a

git-subtree-dir: themes/codex
git-subtree-split: da2d16a4f95fc37e71548dfc139d51e22ebb09bd

static/js/table-of-contents.js

@@ -0,0 +1,17 @@
+window.addEventListener('DOMContentLoaded', () => {
+  const observer = new IntersectionObserver(entries => {
+    entries.forEach(entry => {
+      const id = entry.target.getAttribute('id');
+      if (entry.intersectionRatio > 0) {
+        document.querySelector(`#TableOfContents ul li a[href="#${id}"]`).classList.add('active');
+      } else {
+        document.querySelector(`#TableOfContents ul li a[href="#${id}"]`).classList.remove('active');
+      }
+    });
+  });
+
+  // Track all headers that have an `id` applied
+  document.querySelectorAll('article h3[id], article h2[id]').forEach((section) => {
+    observer.observe(section);
+  });
+});