How can i get rid of the blue highlight inside JCombobox?

The JCombobox's "content" window has this blue highlight that i don't know how to get rid of, please help.

Here is an example of the problem:

package example;

import java.awt.CardLayout;
import java.awt.Color;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Example {

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(1);
        frame.setSize(500, 500);
        frame.setLayout(new CardLayout());
        frame.setVisible(true);
        
        JPanel panel=new JPanel();
        panel.setLayout(null);
        frame.add(panel);

        String[] model = {"pres.", "PPS.", "P. inp.", "P. mais q."};

        JComboBox combox;
        combox = new JComboBox(model);
        combox.setBounds(100, 100, 145, 30);
        combox.setBackground(new Color(215, 211, 165));
        combox.setFocusable(false);
        panel.add(combox);
        
        panel.updateUI();
    }

}

The combo box is a complex components that uses multiple components internally. The UI determines how the components interact with one another.

For example change your code to the following:

JComboBox combox;
combox = new JComboBox(model);
combox.setBorder( new LineBorder(Color.YELLOW) );
BasicComboBoxRenderer renderer = new BasicComboBoxRenderer();
renderer.setBorder( new LineBorder(Color.RED) );
combox.setRenderer(renderer);

And you will notice that the blue highlight is not a border of the combo box or its render, implying there is another internal container we don't have access to.

If you really want to solve the problem then you will need to customize the MetalComboBoxUI class, which is never an easy task because many of the painting methods will be private. But take a look at the code of the class to see if it can be done.

Other issues with your code:

1. Don't use updateUI(). The method is invoked internally in Swing when the LAF is changed. You are not changing the LAF

2. Components should be added to the frame BEFORE the frame is visible. This will eliminate the need for the updateUI().