fix(status_list): Update scroll position after toots are appended (#4277)

app/javascript/mastodon/components/status_list.js

@@ -57,14 +57,15 @@ export default class StatusList extends ImmutablePureComponent {
   componentDidUpdate (prevProps) {
     // Reset the scroll position when a new toot comes in in order not to
     // jerk the scrollbar around if you're already scrolled down the page.
-    if (prevProps.statusIds.size < this.props.statusIds.size &&
+    if (prevProps.statusIds.size < this.props.statusIds.size && this._oldScrollPosition && this.node.scrollTop > 0) {
-        prevProps.statusIds.first() !== this.props.statusIds.first() &&
+      if (prevProps.statusIds.first() !== this.props.statusIds.first()) {
-        this._oldScrollPosition &&
-        this.node.scrollTop > 0) {
         let newScrollTop = this.node.scrollHeight - this._oldScrollPosition;
         if (this.node.scrollTop !== newScrollTop) {
           this.node.scrollTop = newScrollTop;
         }
+      } else {
+        this._oldScrollPosition = this.node.scrollHeight - this.node.scrollTop;
+      }
     }
   }