We recently made all content-thrown popups (alert/confirm) be tab-modal (equiv. app-modal) instead of window-modal (equiv. system-modal). That's great, but we'll need to restore system-modal popups for things like phone apps (bug 674726), where a notification of an incoming phone call really will need to interrupt whatever the user is doing in whatever tab/app. Whether we want to restore the old behavior for trusted apps (phone app), or add new explicitly-system-modal-popup API is an issue for another bug. In any case, we need explicitly-app-modal-popup API. Why? The phone app itself might want to throw an app-modal popup and shouldn't have to worry about whether it's accidentally throwing a system-modal popup.

Popups are app-modal by default, and the "attention screen" ~hack gets us the system-modal behavior.