question: public void installUI ( JComponent c ) { MouseListener [ ] ml = c . getMouseListeners ( ) ; super . installUI ( c ) ; c . setOpaque ( false ) ; for ( int i = 0 ; i < ml . length ; i ++ ) { arrowButton . addMouseListener ( ml [ i ] ) ; } }
target:  Install UI - Set ComboBox opaque. Bug in Metal: arrowButton gets Mouse Events, so add the JComboBox MouseListeners to the arrowButton