QMainWindow and its QDockWidget

[Promethee]

Hi everybody,
I have a simple QMainWindow with a QDockWidget (a tool box) and a central widget (see the joined file called MainWindow1.jpg I don't know how to insert them directly in the text).
When I click on one button of the tool box, I need to change the central widget of the main window :

QPushButton *quit = new QPushButton(tr("&Quit"));
  quit->setFont(QFont("Times", 18, QFont::Bold));
  quit->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Minimum);
 
  delete centralWidget();
  setCentralWidget(quit);
 
  updateGeometry();
  update();

QPushButton *quit = new QPushButton(tr("&Quit"));
  quit->setFont(QFont("Times", 18, QFont::Bold));
  quit->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Minimum);

  delete centralWidget();
  setCentralWidget(quit);

  updateGeometry();
  update();

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But the problem is that the dock widget (the tool box) now takes a half of the main window horizontaly !!!! (see MainWindow2.jpg)

To avoid the resizing of the dock widget I wrote the following code:

setMinimumWidth(m_toolBox->width());
  setMaximumWidth(m_toolBox->width());

setMinimumWidth(m_toolBox->width());
  setMaximumWidth(m_toolBox->width());

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The result was not what I expected : the dock widget is correctly resized but there is a kind of frame taking a half of the main window !!!! (see MainWindow3.jpg)

Somebody have an idea?