The combo boxes look incorrect in JFileChooser. The problem is that SynthFileChooserUIImpl provides custom renderers for the combo boxes. The problem is that these renderers don't look like nimbus combo boxes. The borders and edges all look wrong.
The problem is with the renderers for the combo boxes. Either we need our renderers in SynthComboBoxImpl to delegate to the "normal" renderers, or we need to do some clever work to determine whether the renderer should be opaque or not. I suspect the later is a better solution, though I have tested the former and it certainly works.