One Touch Expandable buttons are missing in the JSplitPane
Run the SwingSet2 in Nimbus Look and Feel and observe the separator. Could check/uncheck the One-Touch Expandable checkbox to verify once it is fixed.
Also change the look and feels, and observer that OneTouchExpandable feature is present in Metal, Windows, Motif and many others. And it is an easy way to hide the otherside of the panel.
Attached is the Nimbus look and feel image of the JSplitPane image from SwingSet2.