[site/ToC][s]: smooth scroll behavior

site/hooks/useHeadingsObserver.js

@@ -1,35 +1,43 @@
 import { useState, useEffect } from "react";
 
-/* Creates an Intersection Observer to keep track of headings intersecting
- * a section of the viewport defined by the rootMargin */
-const getIntersectionObserver = (callback) => {
-  return;
-};
-
 const useHeadingsObserver = () => {
-  const [activeHeading, setActiveHeading] = useState("");
   const [observer, setObserver] = useState(null);
 
+  const activeHeading = "";
+  const timeoutId = null;
+
   /* Runs only after the first render, in order to preserve the observer
-   * between component rerenderings (e.g. after activeHeading change). */
+   * between component rerenderings. */
   useEffect(() => {
     const observer = new IntersectionObserver(
       (entries) => {
-        // entries.forEach((entry) => {
-        //   if (entry.isIntersecting) {
-        //     setActiveHeading(entry.target.id);
-        //   }
-        // });
+        // highlight only the first intersecting heading in the ToC
         const firstIntersectingHeading = entries.find(
           (entry) => entry.isIntersecting
         );
-        if (firstIntersectingHeading) {
-          setActiveHeading(firstIntersectingHeading.target.id);
+        const newActiveHeading = firstIntersectingHeading?.target.id;
+        if (!newActiveHeading || activeHeading === newActiveHeading) {
+          return;
         }
+        if (timeoutId) {
+          clearTimeout(timeoutId);
+        }
+        timeoutId = setTimeout(() => {
+          // remove highlight of the previous active heading in the ToC
+          document
+            .querySelector(`.toc-link[href="#${activeHeading}"]`)
+            ?.classList.remove("active");
+
+          // add highlight to the new active heading in the ToC
+          document
+            .querySelector(`.toc-link[href="#${newActiveHeading}"]`)
+            ?.classList.add("active");
+
+          activeHeading = newActiveHeading;
+        }, 250);
       },
       {
         root: null,
-        threshold: 0.55,
         rootMargin: "-65px 0% -85% 0%", // 65px is a navbar height
       }
     );
@@ -41,21 +49,6 @@ const useHeadingsObserver = () => {
     };
   }, []);
 
-  useEffect(() => {
-    if (!activeHeading) {
-      return;
-    }
-
-    const tocLink = document.querySelector(
-      `.toc-link[href="#${activeHeading}"]`
-    );
-    tocLink.classList.add("active");
-
-    return () => {
-      tocLink.classList.remove("active");
-    };
-  }, [activeHeading]);
-
   return observer;
 };
 

site/styles/global.css

@@ -90,19 +90,25 @@ body {
   @apply p-0;
 }
 
-.toc-link {
-  @apply transition-colors;
-}
-
 /* link (a) element */
-.toc-item .toc-link {
-  @apply text-sm text-slate-400 no-underline break-normal
+.toc-link {
+  @apply transition-colors relative text-sm text-slate-400 no-underline break-normal;
 }
 
-.toc-item .toc-link:not(.active) {
-  @apply hover:text-white
+.toc-link:not(.active) {
+  @apply hover:text-white;
 }
 
 .toc-link.active {
-  @apply text-yellow-li
+  @apply text-yellow-li;
+}
+
+.toc-link::before {
+  position: absolute;
+  content: "";
+  @apply transition-opacity opacity-0 border-l border-yellow-li h-[110%] -top-[5%] -left-[0.5rem];
+}
+
+.toc-link.active::before {
+  @apply opacity-100;
 }