We built something like this for our online 3D modeling/publishing/rendering tool: https://Clara.io

I was just looking if it makes sense to adopt, but our needs are pretty customized compared to what is offered here. But a lot of it is similar. Maybe if we were starting over, this would make sense and we could have adopted it.

Screenshot of our tab/multiple window UI: http://exocortex.com/img/blog/OneYear_ModelingTab.png

I think this is a real contribution though, especially if others build upon it.