Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 21271)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -471,7 +471,8 @@
 @media screen and (min-width: 600px) {
 	body.page-template-full-width-page-php #primary,
 	body.page-template-homepage-php #primary,
-	body.single-attachment #primary {
+	body.single-attachment #primary,
+	body.full-width #primary {
 		width: 100%;
 	}
 }
Index: wp-content/themes/twentytwelve/functions.php
===================================================================
--- wp-content/themes/twentytwelve/functions.php	(revision 21271)
+++ wp-content/themes/twentytwelve/functions.php	(working copy)
@@ -302,12 +302,12 @@
 /**
  * Extends the default WordPress body class to denote a full-width layout.
  *
- * Used in two cases: no active widgets in sidebar, and full-width page template.
+ * Used when there are no active widgets in the sidebar.
  *
  * @since Twenty Twelve 1.0
  */
 function twentytwelve_body_class( $classes ) {
-	if ( ! is_active_sidebar( 'sidebar-1' ) || is_page_template( 'full-width' ) )
+	if ( ! is_active_sidebar( 'sidebar-1' ) )
 		$classes[] = 'full-width';
 
 	return $classes;
@@ -315,12 +315,15 @@
 add_filter( 'body_class', 'twentytwelve_body_class' );
 
 /**
- * Adjust $content width for full-width and single image attachment templates.
+ * Adjust $content width for full-width and single image attachment templates
+ * and when there are no active widgets in the sidebar.
+ * 
+ * @since Twenty Twelve 1.0
  */
 function twentytwelve_content_width() {
-	if ( is_page_template( 'full-width-page.php' ) || is_attachment() ) {
+	if ( is_page_template( 'full-width-page.php' ) || is_attachment() || ! is_active_sidebar( 'sidebar-1' ) ) {
 		global $content_width;
 		$content_width = 960;
 	}
 }
-add_action( 'template_redirect', 'twentytwelve_content_width' );
+add_action( 'template_redirect', 'twentytwelve_content_width' );
\ No newline at end of file