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There appears to be no event that fires when the user toggles their default scroll bar style on 10.7+. It seems like this would be a good candidate for firing the resize event, or maybe some special purpose event if the semantics are wrong.
From what I can tell, no event fires during this, but here's some notes via stack overflow: http://stackoverflow.com/questions/10361100/is-there-a-browser-event-that-fires-when-the-os-x-scroll-bar-style-changes