Fix mw.notify not being visible if first loaded when scrolled down
This regression was caused by using $().offset() on a hidden element, which returns
the current scroll position making the script think the notification area starts
where the scroll position is, so it would not become fixed until you scroll further.
Bug: T114606
Change-Id: Ice62cf3ea7b1a29e33baeca05d1a088a68c9a785
(cherry picked from commit b76f0c0571451672858735fa97e6980dca1741d8)