[4.4] Cassiopeia back to top (#43057)

Author: brianteeman

build/media_source/templates/site/cassiopeia/scss/blocks/_back-to-top.scss

@@ -3,7 +3,7 @@
 */
 
 .back-to-top-link {
-  position: absolute;
+  position: fixed;
   inset-inline-end: 1rem;
   bottom: 1rem;
   z-index: 10000;