Improve table of contents intersection observer (#64)

.changeset/grumpy-trains-decide.md

@@ -0,0 +1,5 @@
+---
+"@astrojs/starlight": patch
+---
+
+Fix table of contents intersection observer for all possible viewport sizes.

packages/starlight/components/TableOfContents/MobileTableOfContents.astro

@@ -121,7 +121,7 @@ const toc = generateToC(Astro.props.headings, config.tableOfContents);
     }
 
     constructor() {
-      super();
+      super({ smallViewport: true });
       const details = this.querySelector('details');
       if (!details) return;
       const closeToC = () => {

packages/starlight/components/TableOfContents/starlight-toc.ts

@@ -12,7 +12,7 @@ export class StarlightTOC extends HTMLElement {
     this._current = link;
   }
 
-  constructor() {
+  constructor({ smallViewport = false } = {}) {
     super();
 
     /** All the links in the table of contents. */
@@ -68,17 +68,35 @@ export class StarlightTOC extends HTMLElement {
       }
     };
 
-    const headingsObserver = new IntersectionObserver(setCurrent, {
-      rootMargin: '-10% 0% -85%',
-    });
-
     // Observe elements with an `id` (most likely headings) and their siblings.
     // Also observe direct children of `.content` to include elements before
     // the first heading.
     const toObserve = document.querySelectorAll(
       'main [id], main [id] ~ *, main .content > *'
     );
-    toObserve.forEach((h) => headingsObserver.observe(h));
+    /** Start intersections at nav height + 2rem padding. */
+    const top = (smallViewport ? 104 : 64) + 32;
+    /** End intersections 1.5rem later. */
+    const bottom = top + 24;
+
+    let observer: IntersectionObserver | undefined;
+    function observe() {
+      if (observer) observer.disconnect();
+      const height = document.documentElement.clientHeight;
+      const rootMargin = `-${top}px 0% ${bottom - height}px`;
+      observer = new IntersectionObserver(setCurrent, { rootMargin });
+      toObserve.forEach((h) => observer!.observe(h));
+    }
+    observe();
+
+    const onIdle = window.requestIdleCallback || ((cb) => setTimeout(cb, 1));
+    let timeout: NodeJS.Timeout;
+    window.addEventListener('resize', () => {
+      // Disable intersection observer while window is resizing.
+      if (observer) observer.disconnect();
+      clearTimeout(timeout);
+      timeout = setTimeout(() => onIdle(observe), 200);
+    });
   }
 }