How do I remove the gap between the edge of JButton lines and the edge of the parent JPanel?

I'm making a calculator with Swing for practice and I can't figure out why I have this gap between the edge of the JButton lines and the edge of the parent JPanel ( buttonPanel

). I looked at my code and changed stuff, but I still can't figure out why the gap exists. If I remove the MatteBorder leaving the default border, the space is gone, but then I have double borders between the buttons. If I reduce the size of my column JTextField

to 7 then the space is gone. How can I eliminate the gap without resorting to double borders or shrinking my application?

(I set the panel background to red to help determine which component is causing the break.)

public class CalculatorView extends JFrame {
    // NOTE: Not recommended to extend JFrame, recommended to use composition over inheritance
    private JTextField display = new JTextField("0", 17);

    // Number buttons
    private JButton zeroButton = new JButton("0");
    private JButton oneButton = new JButton("1");
    private JButton twoButton = new JButton("2");
    private JButton threeButton = new JButton("3");
    private JButton fourButton = new JButton("4");
    private JButton fiveButton = new JButton("5");
    private JButton sixButton = new JButton("6");
    private JButton sevenButton = new JButton("7");
    private JButton eightButton = new JButton("8");
    private JButton nineButton = new JButton("9");
    private JButton periodButton = new JButton(".");

    // Operation buttons
    private JButton additionButton = new JButton("+");
    private JButton subtractionButton = new JButton("-");
    private JButton divisionButton = new JButton("÷");
    private JButton multiplicationButton = new JButton("x");
    private JButton signChangeButton = new JButton("+/-");
    private JButton percentButton = new JButton("%");
    private JButton clearButton = new JButton("AC");
    private JButton equalButton = new JButton("=");

    CalculatorView() {
        // Set the look and feel to the cross-platform look and feel,
        // otherwise mac os will have quirks like gaps between jbuttons
        try {
            UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (Exception e) {
            System.err.println("Unsupported look and feel.");
            e.printStackTrace();
        }
        // Let the OS set location, prevent user from resizing window, and exit app on close
        this.setLocationByPlatform(true);
        this.setResizable(false);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setUndecorated(true);  // must be undecorated for setbackground to work

        // Create the main panel, which by default covers the entire frame
        // NOTE: Good practice. Never put components directly onto a JFrame.
        JPanel gui = new JPanel();
        // Set the main panel layout manager to BorderLayout
        gui.setLayout(new BorderLayout());

        // Create the button panel 
        JPanel buttonPanel = new JPanel();
        // Set button panel layout manager to GridBagLayout
        buttonPanel.setLayout(new GridBagLayout());
        // Create a GridBagConstraints object to control the layout of components
        GridBagConstraints c = new GridBagConstraints();
        c.insets = new Insets(0, 0, 0, 0);
        c.fill = GridBagConstraints.HORIZONTAL;

        // Position buttons on the grid
        c.gridx = 0;
        c.gridy = 0;
        c.ipady = 30;  // adjust vertical height of buttons
        c.weightx = 1; // needed or buttons will cluster at center w/ gap on sides
        buttonPanel.add(clearButton, c);
        clearButton.setBackground(Color.gray);
        clearButton.setOpaque(true);
        clearButton.setBorder(new MatteBorder(0, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(signChangeButton, c);
        signChangeButton.setBackground(Color.gray);
        signChangeButton.setOpaque(true);
        signChangeButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 2; 
        buttonPanel.add(percentButton, c);
        percentButton.setBackground(Color.gray);
        percentButton.setOpaque(true);
        percentButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(divisionButton, c);
        divisionButton.setBackground(Color.ORANGE);
        divisionButton.setOpaque(true);
        divisionButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 1;
        buttonPanel.add(sevenButton, c);
        sevenButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(eightButton, c);
        eightButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(nineButton, c);
        nineButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(multiplicationButton, c);
        multiplicationButton.setBackground(Color.ORANGE);
        multiplicationButton.setOpaque(true);
        multiplicationButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 2;
        buttonPanel.add(fourButton, c);
        fourButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(fiveButton, c);
        fiveButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(sixButton, c);
        sixButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(subtractionButton, c);
        subtractionButton.setBackground(Color.ORANGE);
        subtractionButton.setOpaque(true);
        subtractionButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 3;
        buttonPanel.add(oneButton, c);
        oneButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(twoButton, c);
        twoButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(threeButton, c);
        threeButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(additionButton, c);
        additionButton.setBackground(Color.ORANGE);
        additionButton.setOpaque(true);
        additionButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 4;
        c.gridwidth = 2; // spans 2 cells
        buttonPanel.add(zeroButton, c);
        zeroButton.setBorder(new MatteBorder(1, 1, 1, 1, Color.BLACK));
        c.gridx = 2;
        c.gridwidth = 1; 
        buttonPanel.add(periodButton, c);
        periodButton.setBorder(new MatteBorder(1, 0, 1, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(equalButton, c);
        equalButton.setBackground(Color.ORANGE);
        equalButton.setOpaque(true);
        equalButton.setBorder(new MatteBorder(1, 0, 1, 1, Color.BLACK));

        // Customize display field
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setFont(new Font("Arial", Font.PLAIN, 40));
        display.setBorder(new EmptyBorder(30, 0, 0, 0));
        display.setForeground(Color.WHITE);
        // Make display translucent but leave button panel opaque
        this.setBackground(new Color(0, 0, 0, 100));
        gui.setBackground(new Color(0,0,0,100));
        display.setBackground(new Color(0, 0, 0, 100));

        // I need to fix the gap between the edge of the buttonPanel and the buttons... 
        buttonPanel.setBackground(Color.red);

        // Add display and button panel to main panel, then main panel to frame
        gui.add(display, BorderLayout.NORTH);
        gui.add(buttonPanel, BorderLayout.CENTER);
        this.add(gui);
        this.pack();
    }
}