To reproduce:
-start up SwingSet2
-go to the JEditorPaneHTML Demo tab and click to give it the keyboard focus
-press the down arrow several times. The scrollbar scrolls
-press the PageUp key ONCE. The scroll thumb should go back to the top of the track and stay there, but instead it goes to the top for a brief instant, and then immediately scrolls down by about an inch.

This is reproducible on Solaris/sparc, Linux, and Win32.

Another problem in the same demo:
-move the scroll thumb back to the top of the track
-press the PageDown key once
-repeatedly press the up arrow key. The scroll thumb never makes it all the way back to the top of the track.