QMainWindow or QDialog show() - before displaying, the window area is painted white

[ChrisW67]

Compare what I wrote:

remove the style sheet from Dialog1.ui and modify the palette in Designer so that QPalette::Window is Red.

to what you did in your code:

QPalette pal = palette();
pal.setColor(QPalette::Background, Qt::red);

QPalette pal = palette();
pal.setColor(QPalette::Background, Qt::red);

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You can actually make that palette adjustment to QPalette::Window in Designer if you wish. The QPalette::Window comes from the docs linked earlier.

The other thing I did in your project was apply a layout to everything in your Dialog1. I do not think this will make a difference to the painting only the layout and usability.

[VVS]

You can actually make that palette adjustment to QPalette::Window in Designer

I changed the color of the palette-Window and others in the designer. But it didn't help

[VVS]

I solved this problem.

class Dialog1 : public QDialog
{
    Q_OBJECT
 
public:
    explicit Dialog1(QWidget *parent = nullptr);
    ~Dialog1();
 
private:
    Ui::Dialog1 *ui;
protected:
    bool event(QEvent*e);
};

class Dialog1 : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog1(QWidget *parent = nullptr);
    ~Dialog1();

private:
    Ui::Dialog1 *ui;
protected:
    bool event(QEvent*e);
};

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Dialog1::Dialog1(QWidget *parent) :
    QDialog(parent),ui(new Ui::Dialog1)
{
    ui->setupUi(this);
 
    setWindowFlags(Qt::Window);
    setWindowOpacity(0);
}
 
bool Dialog1::event(QEvent*e)
{
    if(e->type()==QEvent::WindowActivate)
    {
        QTimer::singleShot(0,NULL,[this](){this->setWindowOpacity(1);});
        return true;
    }
    if(e->type()==QEvent::Hide)
    {
        setWindowOpacity(0);
        return true;
    }
    return QDialog::event(e);
}

Dialog1::Dialog1(QWidget *parent) :
    QDialog(parent),ui(new Ui::Dialog1)
{
    ui->setupUi(this);

    setWindowFlags(Qt::Window);
    setWindowOpacity(0);
}

bool Dialog1::event(QEvent*e)
{
    if(e->type()==QEvent::WindowActivate)
    {
        QTimer::singleShot(0,NULL,[this](){this->setWindowOpacity(1);});
        return true;
    }
    if(e->type()==QEvent::Hide)
    {
        setWindowOpacity(0);
        return true;
    }
    return QDialog::event(e);
}

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This is not a Qt problem, this is a Windows problem. I looked at my WPF and Windows Forms projects and saw that they have this problem. You can also see it by running Windows Explorer or Windows Task Manager. Another question is whether this is considered a problem. I think so, Yes. This flickering hits my eyes