Unexpected behaviour when I call show() to show a closed window again

[Mr Pang]

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QMainWindow *mm = new QMainWindow;
    mm->setAttribute(Qt::WA_DeleteOnClose, false);
    connect(ui->pushButton, &QPushButton::clicked, [=]{mm->show();});
}

Hi, I use such code to show the window again. First time to click the button, sub-window popup normally.

Then I close it by up-right "x". I click the button again to show sub-window, and found it stuck with short duration， and then a strange window popup like below

The background is part of my desktop.
When I resize the sub-window. It's background redraw to normal.
I have two question.

1. What may cause the short stuck
2. How to make the background normal by code, not by hand.

Thanks!

[Christian Ehrlicher]

What Qt version do you use? Please check with 5.12.5 since there were problems with 5.12.3/.4 wrt to your problem iirc.

[Mr Pang]

@Christian-Ehrlicher
Oh, Thanks. My version is 5.12.4.
I use another version 5.9.3 and the bug is gone.