Why qt draws sub widget on super bottom not on native nsview

[pengxu]

int main(int argc, char *argv[])
{    
    QApplication a(argc, argv);
    
    QWidget* mw = new QWidget();
    mw->setFixedSize(800, 600);
    NSView* leftView = reinterpret_cast<NSView*>(mw->winId());
    
    QWidget* canvas = new QWidget(mw);
    canvas->setGeometry(0, 0, 800, 600);
    canvas->setStyleSheet("background-color:red");
    
    QWidget* tool = new QWidget(mw);
    tool->setGeometry(0, 400, 800, 600);
    tool->setStyleSheet("background-color:green");
    
   
    QWidget* btn = new QWidget(tool);
    btn->winId();
    btn->setGeometry(0, 0, 100, 100);
    btn->setStyleSheet("background-color:white;");
    
    mw->show();
    return a.exec();
}

why all these red green and white color are drawn on the super bottom nsview? (Inspect it with xcode view hierarchy tool)
If I set tool as native nsview and add layer, it's subview btn is drawn behinde the layer, why?

    NSView* tv = reinterpret_cast<NSView*>(tool->winId());
    [tv setWantsLayer:TRUE];