its happened to me a couple of times already. I make a few real quick changes on my project and hit the "Go to site settings" accidentally, not giving it enough time to backup the changes I made and having to go back in. Or just having to wait for the editor to open up again. Could we include a two step process which ask you to confirm that you want to leave the editor?
Hrm, that's odd - if you have unsynced changes you should see a pop up alert if you tried navigating away from the designer:
Hmmm...at no point did I get a warning. I've actually never seen that window before either. Currently running "Incognito Mode" but other wise always running latest Chrome or Canary.