Ticket #28580: 28580.3.diff

src/wp-admin/js/customize-controls.js

@@ -1850 +1850 @@
                 touchDrag: false,
 
                 /**
+                 * Defer rendering the theme control until the section is displayed.
+                 *
+                 * @since 4.2.0
+                 */
+                renderContent: function () {
+                        var control = this,
+                                renderContentArgs = arguments;
+                        api.section( control.section(), function ( section ) {
+                                if ( section.expanded() ) {
+                                        api.Control.prototype.renderContent.apply( control, renderContentArgs );
+                                } else {
+                                        section.expanded.bind( function ( expanded ) {
+                                                if ( expanded ) {
+                                                        api.Control.prototype.renderContent.apply( control, renderContentArgs );
+                                                }
+                                        } );
+                                }
+                        } );
+                },
+
+                /**
                  * @since 4.2.0
                  */
                 ready: function() {