Bug 853457 - Keep the address bar visible while loading

Fix for [#853457](https://bugzilla.mozilla.org/show_bug.cgi?id=853457). At the moment the address bar becomes visible (including the loading indicator) when you navigate away on a page. However when a scroll event comes in the address bar hides immediately, which is not nice.

This patch takes this into account and keeps the address bar visible until a page is fully loaded, hiding it afterwards if we are passed the scroll threshold.