Tweak forum topic browse menu

The filter box will now remain fixed while the rest of the menu scrolls.
This also fixes a minor cosmetic bug in which the thread list would
overflow its container if the user opened the browse menu, scrolled the
page, and then closed the browse menu.

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

@@ -107,8 +107,9 @@ if Backbone?
 
       headerHeight = @$(".forum-nav-header").outerHeight()
       refineBarHeight = @$(".forum-nav-refine-bar").outerHeight()
+      browseFilterHeight = @$(".forum-nav-browse-filter").outerHeight()
       @$('.forum-nav-thread-list').css('height', (sidebarHeight - headerHeight - refineBarHeight - 2) + 'px')
-      @$('.forum-nav-browse-menu-wrapper').css('height', (sidebarHeight - headerHeight - 2) + 'px')
+      @$('.forum-nav-browse-menu').css('height', (sidebarHeight - headerHeight - browseFilterHeight - 2) + 'px')
 
 
     # Because we want the behavior that when the body is clicked the menu is
@@ -267,6 +268,7 @@ if Backbone?
         @$(".forum-nav-thread-list-wrapper").hide()
         $(".forum-nav-browse-filter-input").focus()
         $("body").bind "click", @hideBrowseMenu
+        @updateSidebar()
 
     hideBrowseMenu: =>
       if @isBrowseMenuVisible()
@@ -274,6 +276,7 @@ if Backbone?
         @$(".forum-nav-browse-menu-wrapper").hide()
         @$(".forum-nav-thread-list-wrapper").show()
         $("body").unbind "click", @hideBrowseMenu
+        @updateSidebar()
 
     toggleBrowseMenu: (event) =>
       event.preventDefault()

lms/static/sass/discussion/elements/_navigation.scss

@@ -64,7 +64,6 @@
 // Browse menu
 // -----------
 .forum-nav-browse-menu-wrapper {
-  overflow-y: scroll;
   border-bottom: 1px solid $gray-l3;
   background: $gray-l5;
 }
@@ -87,6 +86,10 @@
   width: 100%;
 }
 
+.forum-nav-browse-menu {
+  overflow-y: scroll;
+}
+
 .forum-nav-browse-title .icon {
   margin-right: ($baseline/2);
 }