Platform: Mercury with Blue print theme
L&F: GTK
- Open SwingSet2.jar.
- Change LnF to GTK
- Go to file Chooser demo.
- Click on 'Show Plain JFilechooser'.
- Drag split pane from files towards folder, While you are dragging
  you will see a black line in center(horizontally) drawn, This line goes
  away when you finish the drag i.e when you release the pressed mouse.
  For better understanding consider attached image.

PS: This bug is not reproducible in Tiger.