Normally when scrolling a Swing component embedded in a JScrollPane,
AWT/Swing will coalesce unhandled mouse events, this means that if the user and
system is generating the scroll drag events more quickly than Swing can repaint
at the new scroll position, then the unhandled and obsoleted scroll positions
are not used, and Swing skips to the latest scroll position.

But it appears this breaks down if the user's hand is not steady and the
mouse leaves and re-enters the window as s/he drags the mouse.
This is a highly possible scenario since scrollbars tend to be ad the
edge of windows, and you can potentially get many unnecessary repaints.
This was observed on JDK 1.3.1 and 1.4.1 on windows and solaris, but
you need to instrument BasicScrollBarUI to see this.