I finally figured out why despite years of using Windows (i.e. since version 1.0 in the mid-80's, and mouse-oriented apps going back earlier to my Apple //e) I'm having so much trouble using the scroll bars on my relatively new, fancy Windows 8.1 desktop machine at work. I click in the scroll bar and unexpected scrolling happens, or sometimes nothing at all happens.
In this photo with two windows, one is scrolled to the bottom, the other is scrolled to the top.
The problem is the Windows 8.1 default style is for the "thumb" of the scroll bar to be a dark grey, with the empty part to be a light grey, almost white. But one idiotic software vendor decided to override that with their own distinctive style, the "thumb" being white and the empty part of the scroll bar a dark grey.
No, wait... They're both the same software vendor... F*ckin' Microsoft!