Interesting — possibly meaningless — side note: the most significant changes I can spot are whenever Steve Jobs launched a new platform:
- Macintosh (1984): Nearly all of the scrollbars for the next fifteen years look like this one. 
- NeXTstep (1989): Introduced the bar resizing to reflect the amount of content.
- iOS (2007): Introduced dissapearing scroll indicators, a version of which we now have on the Mac as well.
- The Apple Lisa’s scroll bars to me look like an unfinished the proto-version of what showed up on Macintosh. ↩