On Windows, shift+wheel down scrolls left, not right (like it would on the Mac). hyatt tells me this is a bug; it's easy to fix.
Created attachment 28330 [details]
Simple patch to fix. Looks like only Windows and GTK had reversed signs. I'm going to assume GTK was broken like Windows.
Comment on attachment 28330 [details]
Fixed in r41507.
*** Bug 23416 has been marked as a duplicate of this bug. ***
*** Bug 21795 has been marked as a duplicate of this bug. ***