The way we implement it today is we listen to beforeunload events and set the state to detached. This is wrong for 2 reasons:

1. The beforeunload event is cancelable, meaning it's possible for the page to continue to exist after this event fires (e.g. click on a link that downloads a file - issue).
2. The detached state is about the app/engine having no views, and not about the page being unloaded.

To implement this properly, we need to watch the number of views registered in the engine. Once that number drops to zero, we switch to the detached state (and switch back to resumed when a view is registered).

cc @gspencergoog @ditman