diff --git src/wp-includes/class-wp-customize-manager.php src/wp-includes/class-wp-customize-manager.php
index d047620..43ad9e0 100644
--- src/wp-includes/class-wp-customize-manager.php
+++ src/wp-includes/class-wp-customize-manager.php
@@ -1353,6 +1353,9 @@ final class WP_Customize_Manager {
 
 		wp_enqueue_script( 'customize-preview' );
 		add_action( 'wp_head', array( $this, 'customize_preview_loading_style' ) );
+		if ( $this->messenger_channel ) {
+			add_action( 'wp_head', array( $this, 'remove_frameless_preview_messenger_channel' ) );
+		}
 		add_action( 'wp_footer', array( $this, 'customize_preview_settings' ), 20 );
 		add_filter( 'get_edit_post_link', '__return_empty_string' );
 
@@ -1488,6 +1491,34 @@ final class WP_Customize_Manager {
 	}
 
 	/**
+	 * Remove customize_messenger_channel query parameter from the preview window when it is not in an iframe.
+	 *
+	 * @since 4.7.0
+	 * @access public
+	 */
+	public function remove_frameless_preview_messenger_channel( ){
+		if ( ! $this->messenger_channel ) {
+			return;
+		}
+		?>
+		<script>
+		( function() {
+			var urlParser;
+			if ( parent !== window ) {
+				return;
+			}
+			urlParser = document.createElement( 'a' );
+			urlParser.href = location.href;
+			urlParser.search = urlParser.search.replace( /(^|&)customize_messenger_channel=(.+?)(?=$|&)/, '' );
+			if ( urlParser.search !== location.search ) {
+				location.replace( urlParser.href );
+			}
+		} )();
+		</script>
+		<?php
+	}
+
+	/**
 	 * Print JavaScript settings for preview frame.
 	 *
 	 * @since 3.4.0