Fixes slow scrolling when scroll sync is enabled

Some browsers have an animated scroll effect, where it smears
a single scroll event over mutiple smaller scroll events.

Whenever this effect lasted for more than 10 milliseconds
each div's (split editor/preview) scroll even would cancel
the other, which caused scrolling to be really slow.

public/js/user/user.controller.js

@@ -14,17 +14,21 @@ module.exports =
   vm.profile = userService.profile;
 
   var $divs = jQuery('.split-editor, .split-preview');
+  var $allowed = $divs;
   var sync = function(e) {
-    var
-      $other     = $divs.not(this).off('scroll'),
-      other      = $other[0],
-      percentage = this.scrollTop / (this.scrollHeight - this.offsetHeight);
+    var $this = jQuery(this);
 
-    other.scrollTop = Math.round(percentage * (other.scrollHeight - other.offsetHeight));
+    if ($this.is($allowed)) {
+      var
+        other     = $divs.not(this)[0],
+        percentage = this.scrollTop / (this.scrollHeight - this.offsetHeight);
 
-    $timeout(function() {
-      $other.on('scroll', sync);
-    }, 10);
+      other.scrollTop = Math.round(percentage * (other.scrollHeight - other.offsetHeight));
+
+      $allowed = $this;
+    } else {
+      $allowed = $divs;
+    }
 
     return false;
   };