diff --git a/wp-admin/css/wp-admin.dev.css b/wp-admin/css/wp-admin.dev.css
index 7eb6f2d1f6..ab72b262b1 100644
--- a/wp-admin/css/wp-admin.dev.css
+++ b/wp-admin/css/wp-admin.dev.css
@@ -5282,6 +5282,7 @@ body.full-overlay-active {
 	bottom: 0;
 	left: 0;
 	right: 0;
+	height: 100%;
 }
 
 .wp-full-overlay.collapsed,
@@ -5323,6 +5324,7 @@ body.full-overlay-active {
 	right: 0;
 	top: 0;
 	bottom: 0;
+	height: 100%;
 }
 
 .wp-full-overlay.collapsed .wp-full-overlay-main {
diff --git a/wp-admin/js/theme.dev.js b/wp-admin/js/theme.dev.js
index ede9274bd0..dbc521743b 100644
--- a/wp-admin/js/theme.dev.js
+++ b/wp-admin/js/theme.dev.js
@@ -53,6 +53,9 @@ jQuery( function($) {
  * Displays theme previews on theme install pages.
  */
 jQuery( function($) {
+	if( ! window.postMessage )
+		return;
+
 	var preview = $('#theme-installer'),
 		info    = preview.find('.install-theme-info'),
 		panel   = preview.find('.wp-full-overlay-main'),