diff --git a/src/wp-admin/css/customize-nav-menus.css b/src/wp-admin/css/customize-nav-menus.css
index 13308e81f1..002308b569 100644
--- a/src/wp-admin/css/customize-nav-menus.css
+++ b/src/wp-admin/css/customize-nav-menus.css
@@ -589,16 +589,17 @@
 
 #available-menu-items {
 	position: absolute;
-	overflow: hidden;
 	top: 0;
 	bottom: 0;
 	left: -301px;
+	visibility: hidden;
+	overflow: hidden;
 	width: 300px;
 	margin: 0;
 	z-index: 4;
 	background: #eee;
-	-webkit-transition: all 0.2s;
-	transition: all 0.2s;
+	-webkit-transition: left .18s;
+	transition: left .18s;
 	border-right: 1px solid #ddd;
 }
 
@@ -797,6 +798,7 @@ button.not-a-button {
 
 body.adding-menu-items #available-menu-items {
 	left: 0;
+	visibility: visible;
 }
 
 body.adding-menu-items .wp-full-overlay-main {
diff --git a/src/wp-admin/css/customize-widgets.css b/src/wp-admin/css/customize-widgets.css
index 5304d08c2a..278c32fa21 100644
--- a/src/wp-admin/css/customize-widgets.css
+++ b/src/wp-admin/css/customize-widgets.css
@@ -350,10 +350,11 @@ body.adding-widget .add-new-widget:before {
 
 #available-widgets {
 	position: absolute;
-	overflow: auto;
 	top: 0;
 	bottom: 0;
 	left: -301px;
+	visibility: hidden;
+	overflow: auto;
 	width: 300px;
 	margin: 0;
 	z-index: 1;
@@ -435,6 +436,7 @@ body.adding-widget .add-new-widget:before {
 
 body.adding-widget #available-widgets {
 	left: 0;
+	visibility: visible;
 }
 
 body.adding-widget .wp-full-overlay-main {