clean up frontend

app/modifiers/chat-scroll-bottom.js

@@ -0,0 +1,17 @@
+import Modifier from 'ember-modifier';
+
+// Keeps the chat scrolled to the bottom: on insert, when another
+// conversation is opened, and when the number of messages changes. The same
+// conversation and count (e.g. a poll reload) leaves the scroll position
+// alone, so reading history is not interrupted.
+export default class ChatScrollBottom extends Modifier {
+  lastKey = null;
+
+  modify(element, positional) {
+    const [id, count] = positional;
+    const key = `${id}:${count}`;
+    if (key === this.lastKey) return;
+    this.lastKey = key;
+    element.scrollTop = element.scrollHeight;
+  }
+}