Point lastPoint;
jlist.addMouseListener(new MouseAdapter(){
    @Override public void mouseEntered(MouseEvent e){
        lastPoint = e.getPoint();
    }
};
jlist.addMouseMotionListener(new MouseMotionAdapter(){
    @Override public void mouseMoved(MouseEvent e){
        int lastIndex = jlist.locationToIndex(lastPoint);
        Point current = e.getPoint();
        int currentIndex = jlist.locationToIndex(current);
        if (currentIndex == lastIndex) return;

        ... // highlight
        lastPoint = current;
    }
};