Source

File: wp-includes/class-wp-customize-manager.php

public function remove_frameless_preview_messenger_channel() {
		if ( ! $this->messenger_channel ) {
			return;
		}
		?>
		<script>
		( function() {
			var urlParser, oldQueryParams, newQueryParams, i;
			if ( parent !== window ) {
				return;
			}
			urlParser = document.createElement( 'a' );
			urlParser.href = location.href;
			oldQueryParams = urlParser.search.substr( 1 ).split( /&/ );
			newQueryParams = [];
			for ( i = 0; i < oldQueryParams.length; i += 1 ) {
				if ( ! /^customize_messenger_channel=/.test( oldQueryParams[ i ] ) ) {
					newQueryParams.push( oldQueryParams[ i ] );
				}
			}
			urlParser.search = newQueryParams.join( '&' );
			if ( urlParser.search !== location.search ) {
				location.replace( urlParser.href );
			}
		} )();
		</script>
		<?php
	}