Fix being able to scroll away from the loading bar in web UI (#18170)

app/javascript/styles/mastodon/components.scss

@@ -5598,7 +5598,7 @@ a.status-card.compact:hover {
 .loading-bar {
   background-color: $highlight-text-color;
   height: 3px;
-  position: absolute;
+  position: fixed;
   top: 0;
   left: 0;
   z-index: 9999;