http://bugs.winehq.org/show_bug.cgi?id=12094
--- Comment #4 from Anders Aagaard aagaande@gmail.com 2008-05-13 07:04:53 --- Well I dont think mozilla would have accepted the patch unless GTK/QT was moving towards that standard (as the bug claims).
Googling around a bit trying to link it to xorg, but the only thing I found on that was that the xorg-server apple branches atleast has a specific patch for this ( commit 96459ad45daf2e1b4a801c8506274a4da23abc5b "add support for horizontal scrolling (buttons 6 and 7)" ).
So in lack of a better reason, there must be a reason why mozilla accepted it :p