Fixed size windows cannot be restored from full screen mode under macOS Catalina

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
  MainWindow(QWidget *parent = nullptr);
  ~MainWindow();
};
#endif // MAINWINDOW_H

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
  : QMainWindow(parent)
{
  setFixedSize(QSize(200, 100));
  show();
}
MainWindow::~MainWindow()
{
}