Resize panel when component moves to the next line in FlowLayout

 Code to adjust the height of the JPanel to the last component when the Resize event occurs

package test.swing;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

// resize panel
public class MainTest {
    public static void main(String[] args) {
        MainUI mainUI = new MainUI();
        mainUI.setVisible(true);
    }
}

class MainUI extends JFrame {
    FlowLayout flowLayout = new FlowLayout(FlowLayout.LEFT, 0, 0);
    JPanel resizePane = new JPanel(flowLayout);
    JButton testBtn1 = new JButton("TEST 1");
    JButton testBtn2 = new JButton("TEST 2");
    JButton testBtn3 = new JButton("TEST 3");
    JButton testBtn4 = new JButton("TEST 4");
    JButton testBtn5 = new JButton("TEST 5");
    JComponent lastComponent = testBtn5;

    MainUI() {
        setPreferredSize(new Dimension(500, 300));
        setLayout(new BorderLayout());

        resizePane.setBackground(Color.BLUE);
        resizePane.addComponentListener(new ComponentAdapter() {
            Point prevPoint = null;

            @Override
            public void componentResized(ComponentEvent e) {
                super.componentResized(e);
                if (prevPoint == null || prevPoint.y != lastComponent.getLocation().y) {
                    System.out.println("lastComonent moved to " + lastComponent.getLocation());
                    resizePane.setPreferredSize(new Dimension(resizePane.getPreferredSize().width, lastComponent.getLocation().y + lastComponent.getHeight()));
                    resizePane.updateUI();
                }
                prevPoint = lastComponent.getLocation();
            }
        });

        resizePane.add(testBtn1);
        resizePane.add(testBtn2);
        resizePane.add(testBtn3);
        resizePane.add(testBtn4);
        resizePane.add(testBtn5);
        add(resizePane, BorderLayout.NORTH);

        JPanel pane = new JPanel();
        pane.add(new JLabel("TEST TEST"));
        add(pane, BorderLayout.CENTER);

        pack();
    }
}

RUN

Remove gap in FlowLayout in swing

https://docs.oracle.com/en/java/javase/17/docs/api/java.desktop/java/awt/FlowLayout.html

// When creating layout
public FlowLayout(int align, int hgap, int vgap)
new FlowLayout(FlowLayout.LEADING, 0, 0);

// remove horizontal gap
public void setHgap(int hgap)
layout.setHgap(0);

// remove vertical gap
public void setVgap(int vgap)
layout.setVgap(0);