refactor IntersectionObserver to observe viewport in single-column mode (#12735)

app/javascript/mastodon/components/scrollable_list.js

@@ -210,10 +210,13 @@ export default class ScrollableList extends PureComponent {
   }
 
   attachIntersectionObserver () {
-    this.intersectionObserverWrapper.connect({
+    let nodeOptions = {
       root: this.node,
       rootMargin: '300% 0px',
-    });
+    };
+
+    this.intersectionObserverWrapper
+      .connect(this.props.bindToDocument ? {} : nodeOptions);
   }
 
   detachIntersectionObserver () {