如何将非透明切换到透明背景子窗口

Question

#include "trasparentwin.h"
#include <QHBoxLayout>
#include <QWidget>
#include <QLabel>
#include <QMoveEvent>
#include <QPushButton>
#include <QColor>

TrasparentWin::TrasparentWin(QWidget *parent)
:QWidget(parent)
,second_wnd_(nullptr)
,switch_(false)
{
    setFixedSize(400, 400);
    setObjectName("main_window");
    CreateSecondWidget();
    auto switch_button = new QPushButton(this);
    switch_button->move(200, 200);
    switch_button->setText("switch button");
    connect(switch_button, SIGNAL(clicked(bool)), this, SLOT(OnSwitch(bool)));
    setStyleSheet("QWidget#main_window{background-color:gray;}");
}

TrasparentWin::~TrasparentWin()
{

}

void TrasparentWin::CreateSecondWidget()
{
    second_wnd_ = new QWidget(this);
    second_wnd_->setObjectName("second_wnd");
    //second_wnd_->setStyleSheet("QWidget#second_wnd{background-color:gray;}");
    second_wnd_->setWindowFlags(second_wnd_->windowFlags() | Qt::Window | Qt::FramelessWindowHint);
    second_wnd_->setAttribute(Qt::WA_DontCreateNativeAncestors);
    second_wnd_->setAttribute(Qt::WA_NativeWindow);
    second_wnd_->setFixedSize(100, 100);
    auto second_layout = new QHBoxLayout();
    auto text_label = new QLabel();
    text_label->setText("Second Window");
    text_label->setFixedSize(50, 20);
    second_layout->addWidget(text_label, 0, Qt::AlignVCenter);
    second_wnd_->setLayout(second_layout);
    second_wnd_->setVisible(true);
    second_wnd_->setAutoFillBackground(true);
}

void TrasparentWin::moveEvent(QMoveEvent *e)
{
    if (second_wnd_)
    {
        second_wnd_->move(e->pos());
    }
}

void TrasparentWin::OnSwitch(bool checked)
{
    switch_ = !switch_;
    if (switch_)
    {
        QPalette bgpal = second_wnd_->palette();
        bgpal.setColor (QPalette::Background, QColor (255, 255 , 0, 255));
        second_wnd_->setPalette(bgpal);
    }
    else
    {
        QPalette bgpal = second_wnd_->palette();
        bgpal.setColor (QPalette::Background, Qt::transparent);
        second_wnd_->setPalette(bgpal);
    }
}

我首先点击switch_button，second_wnd背景颜色为黄色，然后我再次点击swith_button，即second_wnd背景颜色是黑暗的，但是，这不是我的意图。因为其次点击应该执行以下代码：

QPalette bgpal = second_wnd_->palette();
bgpal.setColor (QPalette::Background, Qt::transparent);
second_wnd_->setPalette(bgpal);

为什么second_wnd背景不透明？ 如何重复透明切换到非透明度