first commit

media/com_categories/css/shared-categories-accordion.css

@@ -0,0 +1,17 @@
+.com-content-categories__item {
+  border-bottom: 1px solid #dfe3e7;
+}
+.com-content-categories__item-title-wrapper {
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: space-between;
+  padding: .5rem 0;
+}
+.com-content-categories__children {
+  flex-basis: 100%;
+  padding-inline-start: 1.5rem;
+  border-top: 1px solid #dfe3e7;
+}
+.com-content-categories__children .com-content-categories__item:last-child {
+  border-bottom: none;
+}

media/com_categories/css/shared-categories-accordion.min.css

@@ -0,0 +1 @@
+.com-content-categories__item{border-bottom:1px solid #dfe3e7}.com-content-categories__item-title-wrapper{-webkit-box-pack:justify;-ms-flex-pack:justify;display:-webkit-box;display:-ms-flexbox;display:flex;-ms-flex-wrap:wrap;flex-wrap:wrap;justify-content:space-between;padding:.5rem 0}.com-content-categories__children{-ms-flex-preferred-size:100%;-webkit-padding-start:1.5rem;border-top:1px solid #dfe3e7;flex-basis:100%;padding-inline-start:1.5rem}.com-content-categories__children .com-content-categories__item:last-child{border-bottom:none}

media/com_categories/joomla.asset.json

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+{
+  "$schema": "https://developer.joomla.org/schemas/json-schema/web_assets.json",
+  "name": "com_categories",
+  "version": "4.0.0",
+  "description": "Joomla CMS",
+  "license": "GPL-2.0-or-later",
+  "assets": [
+    {
+      "name": "com_categories.shared-categories-accordion",
+      "type": "preset",
+      "dependencies": [
+        "com_categories.shared-categories-accordion#style",
+        "com_categories.shared-categories-accordion#script"
+      ]
+    },
+    {
+      "name": "com_categories.shared-categories-accordion",
+      "type": "script",
+      "uri": "com_categories/shared-categories-accordion.min.js",
+      "dependencies": [
+        "core"
+      ],
+      "attributes": {
+        "type": "module"
+      },
+      "version": "39ae85"
+    },
+    {
+      "name": "com_categories.shared-categories-accordion",
+      "type": "style",
+      "uri": "com_categories/shared-categories-accordion.min.css",
+      "version": "b93a2b"
+    }
+  ]
+}

media/com_categories/js/shared-categories-accordion.js

@@ -0,0 +1,40 @@
+/**
+ * @copyright  (C) 2018 Open Source Matters, Inc. <https://www.joomla.org>
+ * @license    GNU General Public License version 2 or later; see LICENSE.txt
+ */
+
+if (!Joomla || !Joomla.Text) {
+  throw new Error('core.js was not properly initialised');
+}
+
+// Selectors used by this script
+const buttonsSelector = '[id^=category-btn-]';
+
+/**
+ * Handle the category toggle button click event
+ * @param event
+ */
+const handleCategoryToggleButtonClick = ({
+  currentTarget
+}) => {
+  const button = currentTarget;
+  const icon = button.querySelector('span');
+
+  // Toggle icon class
+  icon.classList.toggle('icon-plus');
+  icon.classList.toggle('icon-minus');
+
+  // Toggle aria label, aria-expanded
+  const ariaLabel = button.getAttribute('aria-label');
+  const ariaExpanded = button.getAttribute('aria-expanded');
+  button.setAttribute('aria-label', ariaLabel === Joomla.Text._('JGLOBAL_EXPAND_CATEGORIES') ? Joomla.Text._('JGLOBAL_COLLAPSE_CATEGORIES') : Joomla.Text._('JGLOBAL_EXPAND_CATEGORIES'));
+  button.setAttribute('aria-expanded', ariaExpanded === 'false' ? 'true' : 'false');
+  const {
+    categoryId
+  } = button.dataset;
+  const target = document.getElementById(`category-${categoryId}`);
+  target.toggleAttribute('hidden');
+};
+Array.from(document.querySelectorAll(buttonsSelector)).forEach(button => {
+  button.addEventListener('click', handleCategoryToggleButtonClick);
+});

media/com_categories/js/shared-categories-accordion.min.js

@@ -0,0 +1 @@
+if(!Joomla||!Joomla.Text)throw new Error("core.js was not properly initialised");const buttonsSelector="[id^=category-btn-]",handleCategoryToggleButtonClick=({currentTarget:t})=>{const e=t,o=e.querySelector("span");o.classList.toggle("icon-plus"),o.classList.toggle("icon-minus");const a=e.getAttribute("aria-label"),r=e.getAttribute("aria-expanded");e.setAttribute("aria-label",a===Joomla.Text._("JGLOBAL_EXPAND_CATEGORIES")?Joomla.Text._("JGLOBAL_COLLAPSE_CATEGORIES"):Joomla.Text._("JGLOBAL_EXPAND_CATEGORIES")),e.setAttribute("aria-expanded","false"===r?"true":"false");const{categoryId:l}=e.dataset;document.getElementById(`category-${l}`).toggleAttribute("hidden")};Array.from(document.querySelectorAll(buttonsSelector)).forEach((t=>{t.addEventListener("click",handleCategoryToggleButtonClick)}));