fixes back nav button

common/lib/xmodule/xmodule/css/sequence/display.scss

@@ -266,6 +266,7 @@ nav.sequence-nav {
       border: 1px solid #ccc;
       @include linear-gradient(top, #eee, #ddd);
       box-shadow: 0 1px 0 rgba(255, 255, 255, .7) inset;
+      z-index: 1;
 
       &.prev, &.next {
 
@@ -273,7 +274,7 @@ nav.sequence-nav {
           background-position: center;
           background-repeat: no-repeat;
           display: block;
-          height: 34px;
+          height: 100%;
           width: 40px;
           text-indent: -9999px;
           @include transition(all .2s $ease-in-out-quad 0s);