Improve cursor when navigating threads

When the user was at the first comment and scrolling up, the cursor changed from "up" to bidirectional which is inconsistent.

Change-Id: I34fd26d13350ef1593dfea4e6fbae77f5b6a299c

src/Thread.js

@@ -479,7 +479,7 @@ class Thread {
       this.navInitialDirection = Math.sign(steps);
     }
 
-    this.updateCursor(target === this.rootComment ? 0 : Math.sign(steps));
+    this.updateCursor(Math.sign(steps));
 
     return target;
   }