Index: src/wp-content/themes/twentyfifteen/functions.php
===================================================================
--- src/wp-content/themes/twentyfifteen/functions.php	(revision 31053)
+++ src/wp-content/themes/twentyfifteen/functions.php	(working copy)
@@ -266,7 +266,7 @@
 	if ( $next && has_post_thumbnail( $next->ID ) ) {
 		$nextthumb = wp_get_attachment_image_src( get_post_thumbnail_id( $next->ID ), 'post-thumbnail' );
 		$css .= '
-			.post-navigation .nav-next { background-image: url(' . esc_url( $nextthumb[0] ) . '); }
+			.post-navigation .nav-next { background-image: url(' . esc_url( $nextthumb[0] ) . '); border-top: 0; }
 			.post-navigation .nav-next .post-title, .post-navigation .nav-next a:hover .post-title, .post-navigation .nav-next .meta-nav { color: #fff; }
 			.post-navigation .nav-next a:before { background-color: rgba(0, 0, 0, 0.4); }
 		';