diff --git a/src/librustdoc/html/static/rustdoc.css b/src/librustdoc/html/static/rustdoc.css
index 7efe7e1ae3e..679f5f6e3fd 100644
--- a/src/librustdoc/html/static/rustdoc.css
+++ b/src/librustdoc/html/static/rustdoc.css
@@ -179,6 +179,7 @@ nav.sub {
 	top: 0;
 	height: 100vh;
 	overflow: auto;
+	z-index: 1;
 }
 
 .sidebar .current {