Just out of the #triggerComplete code:

• when async action completes (drumroll) async, we send the pulse message from system-context (user's context is already gone)
• on 5.3.x *only*, we call messagesManager.sendLocalMessage(msg), which internally calls MgnlContext.getUser()... on sys-ctx

It should be trivial to remember the user, just like we do on master (in DefaultAsyncActionExecutor since the 5.4.1 refactoring), and pass it along.

1. Let's confirm this suspicion
2. See first raw patch at: https://gist.github.com/mkgl/05e30173624e11e9ee34
   I haven't tested it, but yes it does deserve a test-case; might be worth writing it off master's DefaultAsyncActionExecutorTest first, and then backport it into 5.3's AbstractCommandActionTest.