diff --git src/wp-admin/css/customize-controls.css src/wp-admin/css/customize-controls.css
index 12347f1630..e1a6d27122 100644
--- src/wp-admin/css/customize-controls.css
+++ src/wp-admin/css/customize-controls.css
@@ -2317,6 +2317,10 @@ p.customize-section-description {
 	z-index: -1;
 }
 
+.wp-full-overlay.in-themes-panel.themes-panel-expanded #customize-controls .wp-full-overlay-sidebar-content {
+	overflow: visible;
+}
+
 .wp-customizer .theme-overlay .theme-backdrop {
 	background: rgba( 238, 238, 238, 0.75 );
 	position: fixed;
diff --git src/wp-admin/js/customize-controls.js src/wp-admin/js/customize-controls.js
index 7b275b4d60..458d93accb 100644
--- src/wp-admin/js/customize-controls.js
+++ src/wp-admin/js/customize-controls.js
@@ -3065,13 +3065,17 @@
 					.addClass( 'in-themes-panel' )
 					.delay( 200 ).find( '.customize-themes-full-container' ).addClass( 'animate' );
 
+				_.delay( function() {
+					overlay.addClass( 'themes-panel-expanded' );
+				}, 200 );
+
 				// Automatically open the installed themes section (except on small screens).
 				if ( 600 < window.innerWidth ) {
 					api.section( 'installed_themes' ).expand();
 				}
 			} else {
 				overlay
-					.removeClass( 'in-themes-panel' )
+					.removeClass( 'in-themes-panel themes-panel-expanded' )
 					.find( '.customize-themes-full-container' ).removeClass( 'animate' );
 			}
 		},