下面是我的简化代码示例：

public class TestPanel extends JPanel {

    private static void createAndShowGUI() {
        JFrame myFrame = new JFrame("");
        myFrame.setBounds(new Rectangle(100, 100, 240, 180));
        myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel box = new JPanel(null);
        box.setBackground(Color.BLUE);

        JPanel left = new JPanel();
        left.setOpaque(false);

        JPanel right = new JPanel();
        right.setOpaque(false);

        JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, left, right);
        splitPane.setResizeWeight(0.5);
        splitPane.setOpaque(false);

        box.setLayout(new BorderLayout());
        box.setBorder(new EmptyBorder(10, 10, 10, 10));
        box.add(splitPane);

        myFrame.getContentPane().add(box, BorderLayout.CENTER);
        myFrame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(TestPanel::createAndShowGUI);
    }
}

结果如下：

如果仔细观察左下角，可能会发现奇怪的边界扭曲：

有人知道怎么了吗？在macos上测试，jdk1.8.0\U 251