feat: In-title section search filter for expandable TOC

apps/web/src/components/TitleFilter.svelte

@@ -0,0 +1,95 @@
+<script lang="ts">
+  /**
+   * In-title search filter for the expandable TOC.
+   * Filters sections by keyword, auto-expanding matching chapters
+   * and hiding non-matching sections. Zero-overhead when empty.
+   */
+
+  interface Props {
+    /** CSS selector for the TOC container */
+    tocSelector?: string;
+    /** Placeholder text */
+    placeholder?: string;
+  }
+
+  let { tocSelector = '#title-toc', placeholder = 'Filter sections...' }: Props = $props();
+  let query = $state('');
+  let matchCount = $state(0);
+  let totalCount = $state(0);
+  let debounceTimer: ReturnType<typeof setTimeout> | undefined;
+
+  function applyFilter(q: string): void {
+    const toc = document.querySelector(tocSelector);
+    if (!toc) return;
+
+    const details = toc.querySelectorAll('details');
+    const normalizedQuery = q.toLowerCase().trim();
+    let matches = 0;
+    let total = 0;
+
+    for (const detail of details) {
+      const items = detail.querySelectorAll('li');
+      let chapterHasMatch = false;
+
+      for (const item of items) {
+        total++;
+        const text = item.textContent?.toLowerCase() ?? '';
+        if (!normalizedQuery || text.includes(normalizedQuery)) {
+          (item as HTMLElement).style.display = '';
+          matches++;
+          chapterHasMatch = true;
+        } else {
+          (item as HTMLElement).style.display = 'none';
+        }
+      }
+
+      // Auto-expand chapters with matches, collapse empty ones
+      if (normalizedQuery) {
+        detail.open = chapterHasMatch;
+        (detail as HTMLElement).style.display = chapterHasMatch ? '' : 'none';
+      } else {
+        // Reset: show all, restore default open state
+        (detail as HTMLElement).style.display = '';
+      }
+    }
+
+    matchCount = matches;
+    totalCount = total;
+  }
+
+  $effect(() => {
+    const q = query;
+    if (debounceTimer !== undefined) clearTimeout(debounceTimer);
+    debounceTimer = setTimeout(() => applyFilter(q), 150);
+    return () => {
+      if (debounceTimer !== undefined) clearTimeout(debounceTimer);
+    };
+  });
+</script>
+
+<div class="relative font-sans">
+  <div class="relative">
+    <svg class="pointer-events-none absolute left-2.5 top-1/2 h-3.5 w-3.5 -translate-y-1/2 text-gray-400" fill="none" viewBox="0 0 24 24" stroke="currentColor" stroke-width="2" aria-hidden="true">
+      <path stroke-linecap="round" stroke-linejoin="round" d="M21 21l-6-6m2-5a7 7 0 11-14 0 7 7 0 0114 0z" />
+    </svg>
+    <input
+      type="text"
+      bind:value={query}
+      placeholder={placeholder}
+      class="w-full rounded border border-gray-300 bg-white py-1.5 pl-8 pr-8 text-xs text-gray-900 placeholder-gray-400 focus:border-teal focus:outline-none dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100 dark:placeholder-gray-600 sm:w-72"
+      aria-label="Filter sections within this title"
+    />
+    {#if query}
+      <button
+        class="absolute right-2 top-1/2 -translate-y-1/2 text-gray-400 hover:text-gray-600"
+        onclick={() => { query = ''; }}
+        aria-label="Clear filter"
+      >&times;</button>
+    {/if}
+  </div>
+  {#if query}
+    <p class="mt-1 text-[11px] text-gray-400">
+      {matchCount} of {totalCount} sections match
+    </p>
+  {/if}
+</div>

apps/web/src/pages/browse/[title].astro

@@ -2,6 +2,7 @@
 import { getCollection } from 'astro:content';
 import BaseLayout from '../../layouts/BaseLayout.astro';
 import Breadcrumbs from '../../components/Breadcrumbs.astro';
+import TitleFilter from '../../components/TitleFilter.svelte';
 import { TITLE_NAMES } from '../../data/title-names';
 
 export async function getStaticPaths() {
@@ -73,9 +74,16 @@ const autoExpand = sortedChapters.length <= 10;
     Click a chapter to expand its sections, or click a section to view its full text and change history.
   </p>
 
+  <!-- In-title search filter -->
+  {entries.length > 20 && (
+    <div class="not-prose mt-4">
+      <TitleFilter client:idle placeholder={`Filter ${entries.length} sections...`} />
+    </div>
+  )}
+
   <!-- Expand/Collapse all toggle -->
   {sortedChapters.length > 5 && (
-    <div class="not-prose mt-4 mb-2 flex gap-2 font-sans text-xs">
+    <div class="not-prose mt-3 mb-2 flex gap-2 font-sans text-xs">
       <button
         id="expand-all"
         class="rounded border border-gray-300 px-2 py-1 text-gray-600 hover:bg-gray-50 dark:border-gray-700 dark:text-gray-400 dark:hover:bg-gray-800"