diff --git modules/system/system.base.css modules/system/system.base.css
index 65924bc..78dcbc3 100644
--- modules/system/system.base.css
+++ modules/system/system.base.css
@@ -133,11 +133,16 @@ div.tree-child-horizontal {
 /**
  * TableHeader behavior.
  *
+ * Sticky headers use position: fixed to overlap content. However, fieldsets
+ * using position: relative still overlap sticky headers. To make sticky
+ * headers overlap these fieldsets again, a z-index is needed.
+ *
  * @see tableheader.js
  */
 table.sticky-header {
   background-color: #fff;
   margin-top: 0;
+  z-index: 1;
 }
 
 /**