it means that the user has actually scrolled up manually the view. It's calculated in the "scroll" handler, and it's basically the same calculation you're doing here (see https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/thread_ui.js#L535)

it will be true if the view is currently scrolled up because of the user (because when we append a new message, we will be scrolled up, but not because of the user).

is it clearer ?