fixed grey button on drop

lms/static/coffee/src/discussion/views/discussion_thread_list_view.coffee

@@ -35,7 +35,6 @@ class @DiscussionThreadListView extends Backbone.View
       sidebar.removeClass('fixed');
       sidebar.css('top', '0');
 
-    # sidebarWidth = .32 * $(".discussion-body").width() - 10;
     sidebarWidth = .31 * $(".discussion-body").width();
     sidebar.css('width', sidebarWidth + 'px');
 

lms/static/sass/_discussion.scss

@@ -608,7 +608,7 @@ body.discussion {
           opacity: 0;
         }
 
-        &.dropped {
+        &.is-dropped {
           .browse-topic-drop-btn {
 
             span {
@@ -621,13 +621,13 @@ body.discussion {
         }
       }
 
-      &.dropped {
+      &.is-dropped {
         .browse-topic-drop-btn {
           background-color: #616161;
         }
       }
 
-      &.dropped {
+      &.is-dropped {
         .browse-topic-drop-icon {
           background-position: 0 -16px;
         }