Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 40990)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -5507,6 +5507,13 @@
 			} );
 		} ());
 
+		// Make sure TinyMCE dialogs appear above Customizer UI.
+		$( document ).one( 'wp-before-tinymce-init', function() {
+			if ( ! window.tinymce.ui.FloatPanel.zIndex || window.tinymce.ui.FloatPanel.zIndex < 500001 ) {
+				window.tinymce.ui.FloatPanel.zIndex = 500001;
+			}
+		} );
+
 		api.trigger( 'ready' );
 	});
 
Index: src/wp-admin/js/widgets/text-widgets.js
===================================================================
--- src/wp-admin/js/widgets/text-widgets.js	(revision 40991)
+++ src/wp-admin/js/widgets/text-widgets.js	(working copy)
@@ -130,9 +130,6 @@
 					wp.editor.remove( id );
 				}
 
-				// Adjust z-index for the customizer.
-				window.tinymce.ui.FloatPanel.zIndex = 500001;
-
 				wp.editor.initialize( id, {
 					tinymce: {
 						wpautop: true