Search: Lazy load search JS on first interaction (or after 5s)

This will reduce the QUnit homepage total uncompressed size by
~70 KB from 360KB to 290KB.

_includes/search.html

@@ -12,13 +12,38 @@
 * https://www.jsdelivr.com/package/npm/algoliasearch
 * https://www.jsdelivr.com/package/npm/autocomplete.js
 
-Remember to use the 'defer' attribute.
+We use type="module" as a natural way to cut the mustard,
+executing the script only on modern browsers with ES6 support,
+and causing no errors on older browsers.
+See https://responsivenews.co.uk/post/18948466399/cutting-the-mustard
 {% endcomment %}
-<script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/algoliasearch-lite.umd.js" integrity="sha256-YuyfhK9VEyfW6ejFiPbfOX2SELbFrgFU/8tU3MHuh60=" crossorigin="anonymous"></script>
-<script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/autocomplete.min.js" integrity="sha256-YVWQosorZnr6fALvOW9VALYuInld27RkSPkElGBdCaU=" crossorigin="anonymous"></script>
 <script type="module">
-    // Await deferred scripts
-    window.addEventListener('DOMContentLoaded', function setupSearch() {
+    const loadSearch = () => Promise.all([
+        new Promise((resolve) => document.body.append(Object.assign(document.createElement('script'), {
+            crossOrigin: 'anonymous',
+            integrity: 'sha256-YuyfhK9VEyfW6ejFiPbfOX2SELbFrgFU/8tU3MHuh60=',
+            src: 'https://cdn.jsdelivr.net/npm/[email protected]/dist/algoliasearch-lite.umd.js',
+            onload: resolve
+        }))),
+        new Promise((resolve) => document.body.append(Object.assign(document.createElement('script'), {
+            crossOrigin: 'anonymous',
+            integrity: 'sha256-YVWQosorZnr6fALvOW9VALYuInld27RkSPkElGBdCaU=',
+            src: 'https://cdn.jsdelivr.net/npm/[email protected]/dist/autocomplete.min.js',
+            onload: resolve
+        })))
+    ]);
+    let restoreFocus = false;
+    async function setupSearch(e) {
+        // Remember if first input happened, to restore focus as-needed since
+        // autocomplete.js will replace the input element.
+        // This is before "once" check, to account for input between start and end of preload
+        restoreFocus = restoreFocus || (e && (e.type === 'click' || e.type === 'focus'));
+
+        // Enforce "once" for loading scripts and configuring search field
+        if (setupSearch.setup) return;
+        setupSearch.setup = 1;
+        await loadSearch();
+
         const client = algoliasearch('{{ site.amethyst.algolia.application_id | default: site.algolia.application_id }}', '{{ site.amethyst.algolia.search_only_api_key }}');
         {% if site.amethyst.algolia.sources -%}
         const indexSources = {{ site.amethyst.algolia.sources | jsonify }};
@@ -31,8 +56,10 @@
         const autocompleteOptions = {
             hint: false,
             ariaLabel: 'search input',
-            // Set debug to true if you want to inspect the dropdown
+            // Set debug to true to allow inspecting the dropdown
             debug: true,
+            // Upon focus restore (see below), load suggestions if input is non-empty
+            openOnFocus: true,
             templates: {
                 // https://github.com/algolia/autocomplete.js/issues/248
                 empty(query) {
@@ -77,10 +104,17 @@
                 }
             }
         }));
-        autocomplete('#aa-search-input', autocompleteOptions, autocompleteSources)
+        const $input = autocomplete('#aa-search-input', autocompleteOptions, autocompleteSources)
             .on('autocomplete:selected', (e, suggestion) => location.href = suggestion._href || suggestion.url);
+        if (restoreFocus) $input.focus();
 
         // Disable fallback form
         document.querySelector('#site-search-form').addEventListener('submit', (e) => e.preventDefault());
-    });
+    }
+
+    // Lazyload
+    document.querySelector('#aa-search-input').addEventListener('focus', setupSearch, { once: true });
+    document.querySelector('#site-search-form').addEventListener('click', setupSearch, { once: true });
+    // Preload
+    window.addEventListener('load', () => setTimeout(setupSearch, 7000));
 </script>