Use a class instead of id for horizontal element

src/workspace-view.coffee

@@ -55,7 +55,7 @@ class WorkspaceView extends View
   # Private:
   @content: (state) ->
     @div id: 'workspace', tabindex: -1, =>
-      @div id: 'horizontal', outlet: 'horizontal', =>
+      @div outlet: 'horizontal', =>
         @div id: 'vertical', outlet: 'vertical', =>
           @div outlet: 'panes'
 

static/workspace-view.less

@@ -28,7 +28,7 @@ h6 {
   background-color: @app-background-color;
   font-family: @font-family;
 
-  #horizontal {
+  .horizontal {
     display: -webkit-flex;
     height: 100%;
   }