when offering watchers, only offer those users who are visible

lib/plugins/acts_as_watchable/lib/acts_as_watchable.rb

@@ -32,7 +32,11 @@ module Redmine
 
         # Returns an array of users that are proposed as watchers
         def addable_watcher_users
-          self.project.users.sort - self.watcher_users
+          users = self.project.users.sort - self.watcher_users
+          if respond_to?(:visible?)
+            users.reject! {|user| !visible?(user)}
+          end
+          users
         end
 
         # Adds user as a watcher