When JViewport gets a setViewPosition, it does not verify the position is valid. If scrollRectToVisible is invoked on a component contained in a JViewport the call eventually makes it to the JViewport. Occasionaly JViewport will attempt to scroll a region to visible that shouldn't be scrolled. The code attached illustrates this. Bring up the applet in appletviewer, click on a node in the tree and press page down. Notice the tree moves to the bottom of the pane.
NOTE: This bug was originaly reported by Johann Lisson (###@###.###).