Index: src/wp-includes/widgets/class-wp-widget-custom-html.php
===================================================================
--- src/wp-includes/widgets/class-wp-widget-custom-html.php	(revision 47262)
+++ src/wp-includes/widgets/class-wp-widget-custom-html.php	(working copy)
@@ -47,7 +47,7 @@
 			'customize_selective_refresh' => true,
 		);
 		$control_ops = array(
-			'width'  => 400,
+			'width'  => 370,
 			'height' => 350,
 		);
 		parent::__construct( 'custom_html', __( 'Custom HTML' ), $widget_ops, $control_ops );