In Windows XP Look and Feel Gap betwn JCombo selection rectangle & border
 rectangle is more than in native.
Steps to reproduce:
1. Run SwingSet2 demo.
2. Change to Windows Look And Feel.
3. Choose JComboBox demo.
4. On any of the comboboxes (e.g. Presets), click on the arrow button
   that shows the dropdown list twice (so that list is shown and then closed.)
5. You should see white border around the blue background of the textfield
   of the combobox.
6. Now if you compare this white border gap with any native combo box
   than it is more.

For further clarification consider attached image.
Combo.JPG : It consist on left part Java combo and on right native combo.