Replace jQuery slidedown() function

app/assets/javascripts/application.js.erb

@@ -429,6 +429,18 @@ const replace = (oldElement, newHTMLString) => {
   removeExtraInputs();
 }
 
+const slideDownJS = (element) => {
+  if (element.classList.contains('slide-down'))
+    return;
+
+  element.classList.add('slide-down');
+  const cs = getComputedStyle(element);
+  const paddingHeight = parseInt(cs.paddingTop) + parseInt(cs.paddingBottom);
+  const height = (element.clientHeight - paddingHeight) + 'px';
+  element.style.height = '0px';
+  setTimeout(() => { element.style.height = height; }, 0);
+};
+
 const fetchWithCSRF = (url, params) => {
   let csrfToken = document.querySelector('meta[name="csrf-token"]').getAttribute('content');
   params = params || {};

app/assets/stylesheets/application.css.erb

@@ -1115,9 +1115,13 @@ div#story_box input#story_moderation_reason,
 div#story_box input#story_merge_story_short_id {
 	width: 600px;
 }
-div#story_box .title-reminder {
+#story_box .title-reminder {
 	display: none;
 }
+.slide-down {
+  display: block !important;
+  transition: height .5s ease !important;
+}
 div#story_box #story_tags_a {
 	width: 624px;
 }