[Reg 6.2.2 -> 6.9] QDialog is only responsive if resized to full screen

#include <QApplication>
#include "mainwindow.h"
int main( int argc, char* argv[] )
{
    QApplication app(argc, argv);
    MainWindow wnd; 
    wnd.show();
    return app.exec();
}

#pragma once
#include <QMainWindow>
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget* parent = NULL);
public slots:
    void showDialog();
};

#include "mainwindow.h"
#include <QDialog>
#include <QPushButton>
#include <QVBoxLayout>
#include <QDialogButtonBox>
MainWindow::MainWindow( QWidget* parent ) : QMainWindow(parent)
{
    QPushButton* centralWidget = new QPushButton("Open dialog",this);
    setCentralWidget(centralWidget);
    connect( centralWidget, &QPushButton::clicked, this, &MainWindow::showDialog );
}
void MainWindow::showDialog()
{
    QDialog dlg;
    dlg.setLayout(new QVBoxLayout);
    QDialogButtonBox* box = new QDialogButtonBox(QDialogButtonBox::Ok,&dlg);
    connect( box, &QDialogButtonBox::accepted, &dlg, &QDialog::accept );
    dlg.layout()->addWidget(box);
    QRect geometry = this->geometry();
    dlg.setGeometry( geometry );
    dlg.exec();
}