diff --git a/src/wp-admin/customize.php b/src/wp-admin/customize.php
index 6bf3490812..51db8b3ebb 100644
--- a/src/wp-admin/customize.php
+++ b/src/wp-admin/customize.php
@@ -250,6 +250,7 @@ do_action( 'customize_controls_print_scripts' );
 		$settings['settings'][ $id ] = array(
 			'value'     => $setting->js_value(),
 			'transport' => $setting->transport,
+			'dirty'     => $setting->dirty,
 		);
 	}
 
diff --git a/src/wp-admin/js/customize-controls.js b/src/wp-admin/js/customize-controls.js
index 831a46e240..d5c8746a57 100644
--- a/src/wp-admin/js/customize-controls.js
+++ b/src/wp-admin/js/customize-controls.js
@@ -17,6 +17,7 @@
 
 			this.id = id;
 			this.transport = this.transport || 'refresh';
+			this._dirty = options.dirty || false;
 
 			this.bind( this.preview );
 		},
@@ -2516,7 +2517,8 @@
 		$.each( api.settings.settings, function( id, data ) {
 			api.create( id, id, data.value, {
 				transport: data.transport,
-				previewer: api.previewer
+				previewer: api.previewer,
+				dirty: !! data.dirty
 			} );
 		});
 
diff --git a/src/wp-includes/class-wp-customize-setting.php b/src/wp-includes/class-wp-customize-setting.php
index 5618314728..8bb3e9438b 100644
--- a/src/wp-includes/class-wp-customize-setting.php
+++ b/src/wp-includes/class-wp-customize-setting.php
@@ -60,6 +60,20 @@ class WP_Customize_Setting {
 	public $sanitize_callback    = '';
 	public $sanitize_js_callback = '';
 
+	/**
+	 * Whether or not the setting is initially dirty when created.
+	 *
+	 * This is used to ensure that a setting will be sent from the pane to the
+	 * preview when loading the Customizer. Normally a setting only is synced to
+	 * the preview if it has been changed. This allows the setting to be sent
+	 * from the start.
+	 *
+	 * @since 4.2.0
+	 * @access public
+	 * @var bool
+	 */
+	public $dirty = false;
+
 	protected $id_data = array();
 
 	/**
diff --git a/src/wp-includes/class-wp-customize-widgets.php b/src/wp-includes/class-wp-customize-widgets.php
index 674dee6c99..ef40e2d340 100644
--- a/src/wp-includes/class-wp-customize-widgets.php
+++ b/src/wp-includes/class-wp-customize-widgets.php
@@ -216,6 +216,7 @@ final class WP_Customize_Widgets {
 		$sidebars_widgets = $this->old_sidebars_widgets;
 		$sidebars_widgets = retrieve_widgets( 'customize' );
 		add_filter( 'option_sidebars_widgets', array( $this, 'filter_option_sidebars_widgets_for_theme_switch' ), 1 );
+		unset( $GLOBALS['_wp_sidebars_widgets'] ); // reset global cache var used by wp_get_sidebars_widgets()
 	}
 
 	/**
@@ -332,6 +333,7 @@ final class WP_Customize_Widgets {
 			$setting_id = 'old_sidebars_widgets_data';
 			$setting_args = $this->get_setting_args( $setting_id, array(
 				'type' => 'global_variable',
+				'dirty' => true,
 			) );
 			$this->manager->add_setting( $setting_id, $setting_args );
 		}
@@ -356,6 +358,9 @@ final class WP_Customize_Widgets {
 				$setting_id   = sprintf( 'sidebars_widgets[%s]', $sidebar_id );
 				$setting_args = $this->get_setting_args( $setting_id );
 				if ( ! $this->manager->get_setting( $setting_id ) ) {
+					if ( ! $this->manager->is_theme_active() ) {
+						$setting_args['dirty'] = true;
+					}
 					$this->manager->add_setting( $setting_id, $setting_args );
 				}
 				$new_setting_ids[] = $setting_id;
diff --git a/tests/phpunit/tests/customize/setting.php b/tests/phpunit/tests/customize/setting.php
index 388cf1aefd..51d42801d9 100644
--- a/tests/phpunit/tests/customize/setting.php
+++ b/tests/phpunit/tests/customize/setting.php
@@ -44,6 +44,7 @@ class Tests_WP_Customize_Setting extends WP_UnitTestCase {
 		$this->assertEquals( '', $setting->sanitize_js_callback );
 		$this->assertFalse( has_filter( "customize_sanitize_{$setting->id}" ) );
 		$this->assertFalse( has_filter( "customize_sanitize_js_{$setting->id}" ) );
+		$this->assertEquals( false, $setting->dirty );
 	}
 
 	function test_constructor_with_args() {