Qt5 cann't display QWidget in Windows7

[lvshuchengyin]

I use Qt5 to make a application.

@Mainwindow::Mainwindow(QWidget parent) :
QWidget (parent),
ui(new Ui::Mainwindow)
{
setWindowFlags(Qt::FramelessWindowHint);
setAttribute(Qt::WA_TranslucentBackground,true);
QGraphicsDropShadowEffect eff = new QGraphicsDropShadowEffect(this);
eff->setOffset(0);
eff->setBlurRadius(4);
eff->setColor(QColor(0,0,0,255*0.9);
this->setGraphicsEffect(eff);
ui->setupUi(this);
show();
}@

It is not display in windows 7 except I let it minimize and show it again, it will display normal, but it works fine in windows XP.
And when I use Qt4.8.5 , it worked fine both windows 7 and XP.

How could I correct it?

I do some work according to the google search to try, but has no effect.

@setParent(0); // Create TopLevel-Widget
setAttribute(Qt::WA_NoSystemBackground, true);
setAttribute(Qt::WA_TranslucentBackground, true);
setAttribute(Qt::WA_PaintOnScreen);@

[brcontainer]

A question about this:
@
setAttribute(Qt::WA_PaintOnScreen);
@

return "QWidget::paintEngine: Should no longer be called"?

Qt::WA_PaintOnScreen requires paintEvent

In .h file:
@protected:
virtual void paintEvent(QPaintEvent* evt);@

In .cpp file:
@void Mainwindow::paintEvent(QPaintEvent* evt)
{
...
}@

Probably adding paintEvent will work.