In Chrome/Mac, when my Composer session get signed out (seems to be automatic, every couple of weeks or so...for security purposes?), it gives no indication that I am no longer signed in. Composer continues to "work" allowing me to make changes, and the Save button appears to work.
However, the preview app won't get any of the updates, and if you click "History", that doesn't work. Not until I actually hit refresh on my browser did I see that I had been signed out of Composer. Once I signed in again, I saw that I had lost all work I had "saved" since I was signed out.
Your UI should really check to see if I'm signed in more often. More than once I've made quite a few changes that I thought were saved but that were then lost because it turned out I was signed out. I've got enough challenges to deal with without losing work...hehe.