JSlider has a button like the home plate of baseball.
I found that a horizontal line of that button is not drawn,
when JSlider is used in JColorChooser's RGB panel.
In other test program, that line is drawn correctly.

Please see attached .gif images.
One is of Merlin-b49, and the other of Merlin-b53.
I ran a same program, which is included in Swing-Tutorial. (ColorChooserDemo.java)

This phenomenon occurs since Merlin-b50, both on Solaris and on Windows.