I have a function openFileAction() that is called when I click the 'File' > 'Open' option in my JMenuBar. Its first lines look like this:

private static String myPath = ... // some path

private void openFileAction() {
    JFileChooser fileChooser = new JFileChooser();
    fileChooser.setCurrentDirectory(new File(myPath));
    
    if (fileChooser.showOpenDialog(this) == JFileChooser.APPROVE_OPTION) {
        File f = null;
        try {
            fileChooser.setDialogType(JFileChooser.OPEN_DIALOG);
            fileChooser.setFileFilter(new FileNameExtensionFilter(null, ".txt");
            f = fileChooser.getSelectedFile();
            ...

I only want to see .txt files as suggestions -- so I call setFileFilter() on my fileChooser.

This works fine for the directory fileChooser is set to, myPath -- i.e., in the 'Open' pop-up window that appears, I see only .txt files (and folders) in that directory. However, if I navigate away from myPath in the pop-up window, let's say to Desktop, I see all files (and folders) there, and no longer only the .txt files, as I would like to.

How can I see only .txt files in any directory I navigate to?

First, configure the dialog the way you want it, before you show it, so, instead of...

JFileChooser fileChooser = new JFileChooser();
fileChooser.setCurrentDirectory(new File(myPath));

if (fileChooser.showOpenDialog(this) == JFileChooser.APPROVE_OPTION) {
    File f = null;
    try {
        fileChooser.setDialogType(JFileChooser.OPEN_DIALOG);
        fileChooser.setFileFilter(new FileNameExtensionFilter(null, ".txt");
        f = fileChooser.getSelectedFile();

You should be doing something more like...

JFileChooser fileChooser = new JFileChooser();
fileChooser.setCurrentDirectory(new File(myPath));
fileChooser.setDialogType(JFileChooser.OPEN_DIALOG);
fileChooser.setFileFilter(new FileNameExtensionFilter(null, ".txt");

if (fileChooser.showOpenDialog(this) == JFileChooser.APPROVE_OPTION) {
    File f = null;
    try {
        f = fileChooser.getSelectedFile();

Second, configure the FileFilter correctly. You should be giving it some kind of "description", as this get's presented to the user and you don't need the . in the extension, instead, it should be more like...

fileChooser.setFileFilter(new FileNameExtensionFilter("Text files", "txt");

Runnable example...

import java.awt.EventQueue;
import java.io.File;
import javax.swing.JFileChooser;
import javax.swing.filechooser.FileNameExtensionFilter;

public class Test {
    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                File myPath = new File(".");
                JFileChooser fileChooser = new JFileChooser();
                fileChooser.setCurrentDirectory(myPath);
                fileChooser.setDialogType(JFileChooser.OPEN_DIALOG);
                fileChooser.setFileFilter(new FileNameExtensionFilter("Text files", "txt"));

                if (fileChooser.showOpenDialog(null) == JFileChooser.APPROVE_OPTION) {
                    File selectedFile = fileChooser.getSelectedFile();
                    System.out.println("You have selected " + selectedFile);
                }
            }
        });
    }
}

I'd also consider taking a closer look at How to Use File Choosers