Windows title bar disappears — qbackingstore::flush() called with non-exposed window

I hope you have a wonderful Christmas.

I encountered a problem using QDockWidget:

my essential code:

Code:

---

void iMainWindow::createDockWindows()
     {
       camera = cvCreateCameraCapture(0);
	assert(camera);
 
	QDockWidget *dock = new QDockWidget(tr("2D Camera"), this);
	dock->setAllowedAreas(Qt::RightDockWidgetArea);
	pixmapLabel = new MyCameraWindow(camera, dock);//class MyCameraWindow : public QWidget
	dock->setWidget(pixmapLabel);
	addDockWidget(Qt::RightDockWidgetArea, dock);
}

---

when I run my program, it comes out to be normal:
Attachment 8532

However, if I drag the dock part outside, the windows title bar disappears:
Attachment 8533

Moreover, after dragging the dock widget outside, the command window shows:
qbackingstore::flush() called with non-exposed window
Attachment 8534

How can I keep the windows title bar after dragging the dock widget outside?

Thanks a lot.

Best regards