Optional replace.:

src/snek/static/message-list.js

@@ -277,7 +277,7 @@ class MessageList extends HTMLElement {
 
   upsertMessage(data) {
     let message = this.messageMap.get(data.uid);
-    if (message) {
+    if (message && data.is_final) {
       message.parentElement?.removeChild(message);
     }
     if (!data.message) return;