Tiny problem in using QCanvas/QCanvasView and QWidgetStack

I have a QMainWindow and a QWidgetStack as its central widget. The QWidgetStack contains two widgets which are QTextEdit and QCanvasView. And then I switch the two widgets to show by turns by clicking a pushbutton. The switching performance works well. But when I change the central widget from QTextEdit to the QCanvasView, it appears a two white lines on the canvasview. Since I have not added any code for drawing items, this is very strange. I am confused!
Below is my code:

Code:

#include <qapplication.h>
#include <qstring.h>
#include <qmainwindow.h>
#include <qdockwindow.h>
#include <qtextedit.h>
#include <qtoolbar.h>
#include <qtoolbutton.h>
#include <qpopupmenu.h>
#include <qwidgetstack.h>
#include <qhbox.h>
#include <qlayout.h>
#include <qcanvas.h>
 
class MainWindow:public QMainWindow
{
        Q_OBJECT
public:
        MainWindow(const QString& filename );
        QDockWindow *toolBarDockWindow;
        QWidgetStack *toolBarStack;
        QWidgetStack *centralWidgetStack;
        QToolButton* graphicButton;
        QToolButton* quitButton;
        QTextEdit *textEdit;
        QCanvasView* canvasView;
        QCanvas*     m_canvas;
public slots:
        void switchToolBarMode();
        void switchCentralWidgetMode(int);                                                                                                       
private:
        void init();
        QString m_filename;
        bool m_toolBarMode;
};
 
 
MainWindow::MainWindow(const QString& filename)
        :QMainWindow(0,0,WDestructiveClose)
{
        int screenW;
        int screenH;
        screenW = QApplication::desktop()->width();
        screenH = QApplication::desktop()->height();
        this->resize(screenW,screenH);
        m_filename = QString::null;
        m_toolBarMode = TRUE;
        init();
}
 
void MainWindow::init()
{
        toolBarDockWindow = new
        QDockWindow(QDockWindow::InDock,this,"toolbardockwindow",0);
        toolBarDockWindow->setOrientation(Qt::Horizontal);
        toolBarDockWindow->setHorizontallyStretchable(TRUE);
        toolBarDockWindow->setVerticallyStretchable(TRUE);
        toolBarDockWindow->setMovingEnabled(FALSE);
        this->moveDockWindow(toolBarDockWindow,Qt::DockBottom);
 
        toolBarStack = new QWidgetStack(toolBarDockWindow);
        toolBarDockWindow->setWidget(toolBarStack);
        QWidget* editToolBar = new QWidget(toolBarStack);
        QHBoxLayout* editToolBarLayout = new QHBoxLayout(editToolBar,6,6);
        QWidget* graphicToolBar = new QWidget(toolBarStack);
        QHBoxLayout* graphicToolBarLayout = new QHBoxLayout(graphicToolBar,6,6);
        toolBarStack->addWidget(editToolBar,0);
        toolBarStack->addWidget(graphicToolBar,1);
        graphicButton = new QToolButton(editToolBar,"graphicutton");
        graphicButton->setUsesTextLabel(TRUE);
        graphicButton->setTextLabel("Graphic",FALSE);
        graphicButton->setFont(QFont("Times",20,QFont::Bold));
        editToolBarLayout->addWidget(graphicButton);
 
        QSpacerItem* editToolBarSpacer = new 
        QSpacerItem(0,0,QSizePolicy::Expanding,QSizePolicy::Minimum);
        editToolBarLayout->addItem(editToolBarSpacer);
       quitButton = new QToolButton(graphicToolBar,"quitbutton");
        quitButton->setUsesTextLabel(TRUE);
        quitButton->setTextLabel("Return",FALSE);
        quitButton->setFont(QFont("Times",20,QFont::Bold));
        graphicToolBarLayout->addWidget(quitButton);
 
 
        QSpacerItem* graphicToolBarSpacer = new QSpacerItem(0,0,QSizePolicy::Expanding,QSizePolicy::Minimum);
        graphicToolBarLayout->addItem(graphicToolBarSpacer);
 
        toolBarStack->raiseWidget(0);
connect(graphicButton,SIGNAL(clicked()),this,SLOT(switchToolBarMode()));
        connect(quitButton,SIGNAL(clicked()),this,SLOT(switchToolBarMode()));
 
        centralWidgetStack = new QWidgetStack(this);
        textEdit = new QTextEdit(centralWidgetStack,"textedit");
        textEdit->setTextFormat(Qt::PlainText);
        textEdit->setFocus();
        centralWidgetStack->addWidget(textEdit,0);
 
        canvasView = new QCanvasView(centralWidgetStack,"canvasview");
        m_canvas = new QCanvas(this);
        m_canvas->resize(centralWidgetStack->width(),centralWidgetStack->height());
        m_canvas->setBackgroundColor(black);
        canvasView->setCanvas(m_canvas);
        centralWidgetStack->addWidget(canvasView,1);
        centralWidgetStack->raiseWidget(0);
 
        this->setCentralWidget(centralWidgetStack);
        connect(toolBarStack,SIGNAL(aboutToShow(int)),this,SLOT(switchCentralWidgetMode(int)));
}
 
void MainWindow::switchCentralWidgetMode(int i)
{
        centralWidgetStack->raiseWidget(i);
 
}
 
void MainWindow::switchToolBarMode()
{
        if(m_toolBarMode){
                toolBarStack->raiseWidget(1);
        }else{
                toolBarStack->raiseWidget(0);
        }
        m_toolBarMode = !m_toolBarMode;
}
 
int main(int argc, char *argv[])
{
        QApplication app(argc,argv);
        QString filename = QString::null;
        MainWindow *mainWindow = new MainWindow(filename);
        app.setMainWidget(mainWindow);
        mainWindow->show();
 
        return app.exec();
}
 
#include "mainwindow.moc"

I think the problem is the canvas resize process. Because after I comment off the "m_canvas->resize()"
the white lines disappear. But the Qt's documents say that when constructs a canvas without giving its constructor a width and height arguements, you should resize it before using it. So how should I resize the canvas?

Thanks a lot.