Remove dashed border on collapsed threads

It left a single pixel, which looked like a rendering glitch.

h/static/styles/threads.scss

@@ -29,7 +29,10 @@ $threadexp-width: .6em;
     padding: 0;
     padding-left: $thread-padding;
 
-    &.collapsed > .annotation {
+    &.collapsed {
+      border-color: transparent;
+
+      & > .annotation {
         .body {
           display: none;
         }
@@ -43,6 +46,7 @@ $threadexp-width: .6em;
         }
       }
     }
+  }
 
   .threadexp {
     background: $white;