When I set a QWidget to Qt::FramelessWindowHint, modify the system's primary screen, and then call showMaximized(), the QWidget disappears.

screen is two 1920*1080;
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
setWindowFlags(Qt::FramelessWindowHint );
 
InitWidget();
 
QTimer* ptime = new QTimer(this);
ptime->start(1000);
 
connect(ptime, &QTimer::timeout, this, [&]()

{ qDebug() << "max:" << m_bMainWndMax << " - " << this->size() << "- pos:" << this->geometry(); }

);
}
 
void Widget::slotMaxWindow()

{ m_bMainWndMax = true; QWidget::showMaximized(); }

 
void Widget::slotShowNormalWindow()

{ m_bMainWndMax = false; QWidget::showNormal(); }

 
void Widget::InitWidget()

{ this->setMinimumSize(500, 400);   QVBoxLayout* layout = new QVBoxLayout(this); layout->setSpacing(0); layout->setMargin(0); layout->setContentsMargins(2, 2, 2, 2);   QPushButton* pMaxBtn = new QPushButton(this); pMaxBtn->setText("max"); layout->addWidget(pMaxBtn);   QPushButton* pNormalBtn = new QPushButton(this); pNormalBtn->setText("normal"); layout->addWidget(pNormalBtn);   connect(pMaxBtn, &QPushButton::clicked, this, &Widget::slotMaxWindow); connect(pNormalBtn, &QPushButton::clicked, this, &Widget::slotShowNormalWindow); }

 
int main(int argc, char *argv[])

{     QApplication a(argc, argv);      Widget win;     win.show();       return a.exec(); }