I saw this popup today on all Stack Exchange sites (using Opera 10.61 on Windows XP). Here's a screenshot (click to enlarge):
The Close This Window link executes the code
Update: Looks like this popup is an automatic login notification, at least this is what it says today after I cleared the browser cache. So, the problem with "undefined" was probably due to the cache.
Just FYI here're the steps to see the popup::
- Clear the browser cache, cookies and persistent storage.
- Restart the browser.
- Open stackoverflow.com.
- Log in using Google OpenID.
- Sign in to Google.
- After you return to SO, open another SE site, e.g. meta.stackoverflow.com. Note that you're not logged in here yet.
After ~5 seconds, a popup appears saying that you've been automatically logged in and suggesting to refresh the page: