[site/hooks][s]: simplified observer hook

site/components/Heading.js

@@ -5,13 +5,10 @@ export const Heading = ({ level, observer }) => (props) => {
   useEffect(() => {
     /* start observing heading's intersection with the bounding box
      * set by observer's `rootMargin` */
-    if (observer) {
-      observer.observe(document.getElementById(props.id));
-
-      return () => {
-        observer.unobserve(document.getElementById(props.id));
-      }
+    if (!observer) {
+      return
     }
+    observer.observe(document.getElementById(props.id));
   });
 
   return React.createElement(`h${level}`, {

site/hooks/useHeadingsObserver.js

@@ -4,28 +4,35 @@ 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) => {
-  if (typeof window !== 'undefined') {
-    return new IntersectionObserver(
-      (entries) => {
-        entries.forEach((entry) => {
-          callback(entry);
-        });
-      },
-      {
-        root: null,
-        rootMargin: "-65px 0% -85% 0%" // 65px is a navbar height
-      }
-    );
-  }
+  return new IntersectionObserver(
+    (entries) => {
+      entries.forEach((entry) => {
+        callback(entry);
+      });
+    },
+    {
+      root: null,
+      rootMargin: "-65px 0% -85% 0%" // 65px is a navbar height
+    }
+  );
 };
 
 const useHeadingsObserver = () => {
   const [activeHeading, setActiveHeading] = useState("");
-  const [observer, setObserver] = useState(getIntersectionObserver((entry) => {
+  const [observer, setObserver] = useState(null);
+
+  /* Runs only after the first render, in order to preserve the observer
+   * between component rerenderings (e.g. after activeHeading change). */
+  useEffect((() => {
+    const observer = getIntersectionObserver((entry) => {
       if (entry.isIntersecting) {
         setActiveHeading(entry.target.id);
       }
-    }));
+    });
+    setObserver(observer);
+
+    return observer.disconnect();
+  }), [])
 
   /* On initial render activeHeading will be `null`, since the observer
    * has not been instantiated yet. However, we still want to highlight