title: QT4 using QMdiArea and QScrollArea strange usage trouble
body: <p>Here is what I am doing： mainwindow with MdiArea, and I add a scrollarea widget (which contains a image label) to MdiArea as a subwindow. It doesn't work (the picture doesn't show).</p>
  
  <p>Here is my code:</p>
  
  <pre><code>MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
  {
    ui-&gt;setupUi(this);
  
    QScrollArea sa;
    QPixmap *image = new QPixmap("2.jpg");
    QLabel* imageLabel = new QLabel();
    imageLabel-&gt;setPixmap(*image);
    sa.setWidget(imageLabel);
    sa.show();
    ui-&gt;mdiArea-&gt;addSubWindow(&amp;sa);
  }
  </code></pre>
  
  <p>But when I use a <code>QLabel</code> as subwindow directly, i.e. replace the last line with:</p>
  
  <pre><code>ui-&gt;mdiArea-&gt;addSubWindow(imageLabel);
  </code></pre>
  
  <p>it works perfectly.</p>
  
  <p>Anyone know why this is happening?</p>