Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 58784)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -2931,6 +2931,11 @@
 		height: 45px;
 	}
 
+	.preview-only #customize-preview iframe {
+		padding-top: 46px; /* #customize-controls height plus 1px border */
+		box-sizing: border-box;
+	}
+
 	.preview-only #customize-preview,
 	.preview-only .customize-controls-preview-toggle .controls {
 		display: block;