JAVA - JTextArea has a fake visual location after translate its parent graphics

I suggest the following for draggable text areas:

1. Do not override paintComponent or any painting method, don't use AffineTransforms or anything that moves pixels around. Instead move the components themselves.
2. This might be one of the few times that I might use a null layout. Note that there are layout managers that can be used in its place (and I think Camickr has written code for this and may be able to give you a link to this).
3. Never set the size of the JTextArea or even its preferred size.
4. Instead, set the column and row properties.
5. Add the JTextArea to a JScrollPane (to its viewport actually)
6. Add a MouseListener and MouseMotionListener to the JScrollPane that moves it
7. Add a MouseListener and MouseMotionListener to the JTextArea that forwards mouse actions to the JScrollPane that holds it, but be sure to change the MouseEvent's source property to reflect the JScrollPane, not the JTextArea.

For example:

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.*;

@SuppressWarnings("serial")
public class DraggingBoxesPanel extends JPanel {
    public DraggingBoxesPanel() {
        int w = 900;
        int h = 600;
        setPreferredSize(new Dimension(w, h));

        setLayout(null); // I usually avoid doing this
        add(createTextBox()); // add first "box" to this JPanel
    }

    // exposed method to allow adding a new "box" to GUI
    public void newBox() {
        add(createTextBox());
        revalidate(); // The JScrollPane requires this
        repaint();
    }

    private JComponent createTextBox() {
        int rows = 10; // jtextarea property
        int cols = 20; // jtextarea property

        // create text area and scroll pane
        JTextArea textArea = new JTextArea(rows, cols);
        JScrollPane scrollPane = new JScrollPane(textArea);

        // let scrollpane size itself
        scrollPane.setSize(scrollPane.getPreferredSize());

        // scrollpane's mouse listeners
        MyMouse myMouse = new MyMouse();
        scrollPane.addMouseListener(myMouse);
        scrollPane.addMouseMotionListener(myMouse);

        // mouse adapter that forwards mouse actions in
        // text area into its containing scrollpane
        MouseAdapter textAreaMouseAdapter = new MouseAdapter() {
            @Override
            public void mouseReleased(MouseEvent e) {
                // make sure source is correct
                e.setSource(scrollPane);
                myMouse.mouseReleased(e);
            }

            @Override
            public void mousePressed(MouseEvent e) {
                e.setSource(scrollPane);
                myMouse.mousePressed(e);
            }

            @Override
            public void mouseDragged(MouseEvent e) {
                e.setSource(scrollPane);
                myMouse.mouseDragged(e);
            }
        };

        textArea.addMouseListener(textAreaMouseAdapter);
        textArea.addMouseMotionListener(textAreaMouseAdapter);

        return scrollPane;
    }

    // code that allows a component to be dragged
    private class MyMouse extends MouseAdapter {
        private Point mousePt1;
        private Point compPt1;

        @Override
        public void mousePressed(MouseEvent e) {
            mousePt1 = e.getLocationOnScreen();
            Component comp = (Component) e.getSource();
            compPt1 = comp.getLocation();
            
            Container container = comp.getParent();
            container.setComponentZOrder(comp, 0);
            container.revalidate();
            container.repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (mousePt1 == null) {
                return;
            }
            dragComponent(e);
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            dragComponent(e);
            mousePt1 = null;
        }

        private void dragComponent(MouseEvent e) {
            Point mousePt2 = e.getLocationOnScreen();
            int x = compPt1.x + mousePt2.x - mousePt1.x;
            int y = compPt1.y + mousePt2.y - mousePt1.y;
            Component comp = (Component) e.getSource();
            comp.setLocation(x, y);
            
            Container container = comp.getParent();
            container.revalidate();
            container.repaint();
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }

    private static void createAndShowGui() {
        final DraggingBoxesPanel mainPanel = new DraggingBoxesPanel();
        JButton newBoxBtn = new JButton("New Box");
        newBoxBtn.addActionListener(e -> mainPanel.newBox());
        JPanel bottomPanel = new JPanel();
        bottomPanel.add(newBoxBtn);

        JFrame frame = new JFrame("DraggingBoxesPanel");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.add(mainPanel);
        frame.add(bottomPanel, BorderLayout.PAGE_END);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

I found a solution to this problem.

In the void paintComponent(Graphics g), instead of

Graphics2D g2 = (Graphics2D) g;

using

Graphics2D g2 = (Graphics2D) g.create();

Solve the problem, but don't know why.

I'm new in JAVA, so I cannot deeply understand all the warnings from everyone's advice, but thanks a lot anyway!