ComboBox dropdown border is unlike native GTK.
Steps to reproduce:
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1) open SwingSet2 and go to ComboBox Demo. Change the LAF to GTK.
2) Click on the ComboBox.
3) Notice the border of the dropdown.
4) Now Click on the action button on the task bar and click the Run Application.
5) You will see a window with a Combobox and some buttons.
6) Click the ComboBox and notice the border. The Border is unlike the Java implementation. It is much more embossed.

The Machine info:
Sol10, JDS release 3, sparc