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:

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:



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


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


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

Thanks a lot.

Best regards --------------------编程问答-------------------- hey, anyone could give some help?

thanks

补充：移动开发 ,  Qt