Issue with overlay scrollbars on OS X in some situations

OS X disables the overlay scrollbars by default on certain machines, e.g. older MacBooks. However if you connect an external device like a Magic Mouse, OS X switches to the overlay scrollbar style. Sublime Text doesn't handle this situation properly: scrollbars do not appear when scrolling using my Macbook's trackpad, whereas they do when using the Magic Mouse.

Steps to reproduce:

1. Attach a newfangled pointing device, e.g. a Magic Mouse, to your crusty old MacBook. OS X should switch to the overlay scrollbar style; the scrollbars will be hidden by default.

2. In Sublime Text, perform a two-finger scroll on the MacBook's trackpad.

Expected behaviour:

The scrollbars appear when you begin the two-finger scroll, and disappear shortly after you finish it.

Actual behaviour:

When you scroll using the trackpad, Sublime doesn't display scrollbars at all, leaving you rather confused as to your position within the current file. If you scroll using the Magic Mouse, the scrollbars do appear.