make cw-tab a positioned element, fixes #804

resources/assets/stylesheets/scss/courseware.scss

@@ -451,6 +451,7 @@ $consum_ribbon_width: calc(100% - 58px);
                     >.cw-tab {
                         max-height: 700px;
                         padding: 14px 8px 8px 8px;
+                        position: relative;
                         overflow-y: auto;
                         overflow-x: hidden;
                         scrollbar-width: thin;

resources/vue/components/courseware/CoursewareRibbonToolbar.vue

@@ -151,7 +151,7 @@ export default {
             setTimeout(() => {
                 let contents = this.$refs.contents.$el; 
                 let current = contents.querySelector('.cw-tree-item-link-current');
-                contents.scroll({ top: current.offsetTop, behavior: 'smooth' });
+                contents.scroll({ top: current.offsetTop - 4, behavior: 'smooth' });
             }, 360);
         },
     },