Given the task T2958 you should be able to fire another event:
when the user is going to click on the preview tab.
This is important because when the event occurs the offline parsing should start
This is the Commit for alerting user when he clicks the preview button when he's offline!
Ok, it works for me!
This is the latest commit for displaying the log (console.log) to the user when he clicks the wikieditor's preview button when he's offline!