add: Extract ReplyPlaceholderPreview from ReplyPlaceholder

js/src/forum/components/ReplyPlaceholderPreview.js

@@ -0,0 +1,36 @@
+/*global s9e*/
+
+export default class ReplyPlaceholderPreview {
+  view() {
+    return <div className="Post-body" />;
+  }
+
+  oncreate(vnode) {
+    // Every 50ms, if the composer content has changed, then update the post's
+    // body with a preview.
+    let preview;
+    this.updateInterval = setInterval(() => {
+      // Since we're polling, the composer may have been closed in the meantime,
+      // so we bail in that case.
+      if (!app.composer.isVisible()) return;
+
+      const content = app.composer.fields.content();
+
+      if (preview === content) return;
+
+      preview = content;
+
+      const anchorToBottom = $(window).scrollTop() + $(window).height() >= $(document).height();
+
+      s9e.TextFormatter.preview(preview || '', vnode.dom);
+
+      if (anchorToBottom) {
+        $(window).scrollTop($(document).height());
+      }
+    }, 50);
+  }
+
+  onremove(vnode) {
+    clearInterval(this.updateInterval);
+  }
+}