xcb: no activeWindow after closing a modal dialog

#include <QApplication>
#include <QDebug>
#include <QMessageBox>
#include <QPushButton>
#include <QThread>
#include <QVBoxLayout>
#include <QWidget>

int main(int argc, char * argv[]) {
	QApplication qapp(argc, argv);
	QWidget widget;
	widget.setLayout(new QVBoxLayout);
	QPushButton button;
	widget.layout()->addWidget(&button);
	widget.show();
	QObject::connect(&button, &QPushButton::clicked, [&widget](){
		qDebug() << QApplication::activeWindow();
		QMessageBox box;
		box.exec();
		qDebug() << QApplication::activeWindow();
		QCoreApplication::processEvents();
		widget.activateWindow();// QThread::msleep(10);// works also
		QCoreApplication::processEvents();
		qDebug() << QApplication::activeWindow();
		exit(0);
	});
	return qapp.exec();
}