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

[superwave]

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:
!http://www.qtforum.org/wcf/images/photos/thumbnails/medium/photo-13-aea5aacc.jpg(before dragging the dock)!

However, if I drag the dock part outside, the windows title bar disappears:
!http://www.qtforum.org/wcf/images/photos/thumbnails/medium/photo-14-84e7543e.jpg(After dragging the dock)!

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

!http://www.qtforum.org/wcf/images/photos/thumbnails/medium/photo-15-c91f04d1.jpg(The command window)!

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

Thanks a lot.

Best regards