* Open a tab with your site in it
* In a new tab, open WP and edit template file, using ACE
* Make an edit of some kind and save
* Immediately switch to the site tab, ie. go preview your edits
If you do the tab switch before the 5sec dialog closes, once it's done it will snatch focus back to the WP editor.
This happens across Windows and OSX, in Safari only; not Chrome, so doesn't appear to be WebKit. Also tested Windows IE9 and Firefox 17 with no duplication.