zenno: make article toc only render upon first view

zenno/src/routes/doc/Article.svelte

@@ -10,6 +10,26 @@
 
 	let { head, children }: Props = $props();
 
+	let tocViewed = $state(false);
+	let tocElem: HTMLElement | undefined = $state();
+	const observeTOC: IntersectionObserverCallback = (entries, obs) => {
+		for (const e of entries) {
+			if (!e.isIntersecting) {
+				continue;
+			}
+			tocViewed = true;
+			obs.disconnect();
+		}
+	}
+
+	$effect(() => {
+		if (tocElem == null) {
+			return;
+		}
+		const obs = new IntersectionObserver(observeTOC, {rootMargin: "20% 0px"});
+		obs.observe(tocElem);
+	});
+
 	function tocClass(s: Section) {
 		switch (s.h) {
 			case 1:
@@ -32,10 +52,12 @@
 	{@render head()}
 	{#if sections.length > 0}
 		<Sec h={2} id="toc" toc={false}>Table of Contents</Sec>
-		<ul class="mb-4 list-disc pl-4">
+		<ul class="mb-4 list-disc pl-4" bind:this={tocElem}>
+			{#if tocViewed}
 				{#each sections as s (s.id)}
 					<li class={tocClass(s)}><a href={`#${s.id}`}>{@render s.children()}</a></li>
 				{/each}
+			{/if}
 		</ul>
 	{/if}
 	{@render children()}