Did you see the jpgs that I uploaded before?
I've already test the code for many times and the different between them is that if I set the transparency attribute of the top-level window. If it does then part of the child widgets(e.g through the QTabWidget's tab part we can see the desktop ) will show unexpected results.
Hers is my code:
Qt Code: 
  1. DDialog::DDialog(QWidget* parent)
  2.         :QWidget(parent, Qt::FramelessWindowHint | Qt::Dialog)
  3. {
  4.     //this->setAutoFillBackground(true);    //if it is set the top-level window was not transparent
  5.                                             //if not, the child widget was not show correctly
  6.     this->setAttribute(Qt::WA_TranslucentBackground);
  7.     this->move(50, 50);
  8.     this->resize(420, 430);
  9.     this->Initialize();
  10. }
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Qt Code: 
  1. void DDialog::Initialize()
  2. {
  3.     QVBoxLayout* layout = new QVBoxLayout(this);
  4.     this->setLayout(layout);
  5.     layout->setMargin(0);
  6.     layout->setSpacing(0);
  7.     //TitleBar ZONE
  8.     this->m_titleBar = new DTitleBar(this);
  9.     layout->addWidget(m_titleBar);
  10.  
  11.     this->m_mainWidget = new QWidget();
  12.     layout->addWidget(m_mainWidget);
  13.     m_mainWidget->setAutoFillBackground(true);
  14.  
  15.     QStatusBar* statusBar = new QStatusBar();
  16.     layout->addWidget(statusBar);
  17.     statusBar->setAutoFillBackground(true);
  18.     QVBoxLayout* centerLayout = new QVBoxLayout();
  19.     this->m_mainWidget->setLayout(centerLayout);
  20.  
  21.     //TabWidget ZONE
  22.     this->m_tabWidget = new QTabWidget();
  23.     this->m_tabWidget->setAttribute(Qt::WA_OpaquePaintEvent);
  24.     centerLayout->addWidget(this->m_tabWidget);
  25.     centerLayout->setMargin(10);
  26.     this->m_tabWidget->setAutoFillBackground(true);
  27.  
  28.     m_tabWidget->setGeometry(QRect(20, 20, 381, 361));
  29.     this->m_tabWidget->setTabPosition(QTabWidget::North);
  30.     this->m_tabWidget->setTabShape(QTabWidget::Rounded);
  31.     this->m_tabWidget->setElideMode(Qt::ElideMiddle);
  32.  
  33.     //TableView ZONE
  34.     this->m_bmpTableHView = new QTableView();
  35.     this->m_bmpTableIView = new QTableView();
  36.     this->m_bmpTableHView->setAutoFillBackground(true);
  37.     this->m_bmpTableIView->setAutoFillBackground(true);
  38.     this->m_bmpTableHView->setGeometry(QRect(-10, -10, 391, 351));
  39.     this->m_bmpTableHView->setFrameShadow(QFrame::Sunken);
  40.     this->m_tabWidget->addTab(this->m_bmpTableHView, "BitMapHead");
  41.     this->m_tabWidget->addTab(this->m_bmpTableIView, "BitMapInfoHead");
  42.  
  43.     //QStandardItemModel ZONE
  44.     this->m_tableHModel = new QStandardItemModel(4, 3, this);
  45.     this->m_tableHModel->setHeaderData(0, Qt::Horizontal, tr("Name"));
  46.     this->m_tableHModel->setHeaderData(1, Qt::Horizontal, tr("Bytes"));
  47.     this->m_tableHModel->setHeaderData(2, Qt::Horizontal, tr("Value"));
  48.     this->m_bmpTableHView->setModel(this->m_tableHModel);
  49.  
  50.     this->m_tableIModel = new QStandardItemModel(4, 3, this);
  51.     this->m_tableIModel->setHeaderData(0, Qt::Horizontal, tr("Name"));
  52.     this->m_tableIModel->setHeaderData(1, Qt::Horizontal, tr("Bytes"));
  53.     this->m_tableIModel->setHeaderData(2, Qt::Horizontal, tr("Value"));
  54.     this->m_bmpTableIView->setModel(this->m_tableIModel);
  55.  
  56.     //BUTTON ZONE
  57.     QHBoxLayout* buttomLayout = new QHBoxLayout();
  58.     centerLayout->addLayout(buttomLayout);
  59.     this->m_OK = new QPushButton();
  60.     this->m_OK->setGeometry(QRect(230, 390, 77, 25));
  61.     this->m_OK->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
  62.     this->m_OK->setText(tr("OK"));
  63.     buttomLayout->addWidget(m_OK, 0, Qt::AlignRight);
  64.  
  65.     this->m_Cancel = new QPushButton();
  66.     this->m_Cancel->setGeometry(QRect(310, 390, 77, 25));
  67.     this->m_Cancel->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
  68.     buttomLayout->addWidget(m_Cancel);
  69.     this->m_Cancel->setText(tr("Cancel"));
  70. }
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