While investigating the failures of some focus related tests I have found that the next simple usecase intermittently fails:

$Xvfb :10 -screen 0 1920x1080x24 -fbdir /tmp &
$export DISPLAY=:10
$mwm &
$x=1; while ............/jdk/jdk-21/bin/java MoveFocusBackToParent ; do echo "ITERATION" $(( x++ )); done

It fails 3~5 times per 100 runs.

import java.awt.*;

public final class MoveFocusBackToParent {

    public static void main(String[] args) throws Exception {
        Frame parent = new Frame();
        Dialog child = new Dialog(parent);
        try {
            parent.setSize(300, 300);
            parent.setBackground(Color.GREEN);
            parent.setLocationRelativeTo(null);
            parent.setVisible(true);
            Thread.sleep(1000); // sleep is used to eliminate any other issues
            child.setSize(300, 300);
            child.setBackground(Color.RED);
            child.setLocationRelativeTo(parent);
            child.setVisible(true);
            Thread.sleep(1000);
            child.toFront();
            Thread.sleep(1000);
            child.requestFocus();
            Thread.sleep(1000);

            child.setVisible(false);
            Thread.sleep(1000);

            if (!child.isFocused()) {
                throw new Error("Child is not focused");
            }
            if (child.isFocused()) {
                throw new Error("Child is focused");
            }
            if (!parent.isFocused()) {
                throw new Error("Parent is not focused");
            }
        } finally {
            child.dispose();
            parent.dispose();
        }
    }
}