Bad geometry for widgets created with QWidget::createWindowContainer(QWindow::fromWinId())

#include <QProcess>
#include <QVBoxLayout>
#include <QLabel>
#include <QMessageBox>
#include <QTextStream>
#include <QToolButton>
#include <QWindow>

int main(int argc, char** argv)
{
    QApplication qapp(argc, argv);

    if (qapp.arguments().contains("--slave")) {
        QWidget     slaveWindow;
        QVBoxLayout layout(&slaveWindow);
        QToolButton button;

        button.setText("Click Me");
        button.connect(&button, &QToolButton::clicked, [&slaveWindow]() {
            QMessageBox::information(&slaveWindow, "Close Me", "Resize the mainWindow once this dialog is closed.", QMessageBox::Ok);
        });
        layout.addWidget(new QLabel("slaveWindow", &slaveWindow));
        layout.addWidget(&button);
        slaveWindow.setStyleSheet("background: red;");
        slaveWindow.show();

        QTextStream ostream(stdout);
        ostream << slaveWindow.winId();
        ostream.flush();

        return qapp.exec();
    } else {
        QWidget     mainWindow;
        QVBoxLayout layout(&mainWindow);
        QProcess    process;
        WId         processWid;

        process.start(qapp.applicationFilePath(), {"--slave"});
        process.waitForReadyRead();
        processWid = process.readAll().toUInt();

        QWindow* window = QWindow::fromWinId(processWid);
        QWidget* widget = QWidget::createWindowContainer(window);
        layout.addWidget(new QLabel("mainWindow", &mainWindow));
        layout.addWidget(widget);
        mainWindow.setStyleSheet("background: green;");
        mainWindow.resize(400, 400);
        mainWindow.show();

        return qapp.exec();
    }
}