why scrollbar doesn't display in QscrollArea

[myhot21]

i add Multiple QListwidget to QscrollArea,but the scrollbar doesn't display in QscrollArea, why?

#include "mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    rightLayout = new QVBoxLayout;
    QWidget *widget = new QWidget;
    widget->setLayout(rightLayout);
    widget->setMinimumHeight(100);
    widget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
 
    QScrollArea *scroll = new QScrollArea;
    scroll->setWidget(widget);
    scroll->setWidgetResizable(true);
    splitter = new QSplitter;
    QPushButton *button = new QPushButton;
    button->setText("Add Widget");
    button->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
    splitter->addWidget(button);
 
    splitter->addWidget(scroll);
 
    QObject::connect(button, SIGNAL( clicked() ), this, SLOT(addMyWidget()));
 
 
    this->setCentralWidget(splitter);
}
void MainWindow::addMyWidget()
{
    QListWidget *listWidget = new QListWidget;
    for(int i = 0; i < 4; i++)
    {
        QListWidgetItem *newItem = new QListWidgetItem;
        newItem->setText("aaaaaa");
        listWidget->addItem(newItem);
    }
 
    listWidget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    rightLayout->addWidget(listWidget);
}
 
MainWindow::~MainWindow()
{
 
}

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    rightLayout = new QVBoxLayout;
    QWidget *widget = new QWidget;
    widget->setLayout(rightLayout);
    widget->setMinimumHeight(100);
    widget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

    QScrollArea *scroll = new QScrollArea;
    scroll->setWidget(widget);
    scroll->setWidgetResizable(true);
    splitter = new QSplitter;
    QPushButton *button = new QPushButton;
    button->setText("Add Widget");
    button->setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Fixed);
    splitter->addWidget(button);

    splitter->addWidget(scroll);

    QObject::connect(button, SIGNAL( clicked() ), this, SLOT(addMyWidget()));


    this->setCentralWidget(splitter);
}
void MainWindow::addMyWidget()
{
    QListWidget *listWidget = new QListWidget;
    for(int i = 0; i < 4; i++)
    {
        QListWidgetItem *newItem = new QListWidgetItem;
        newItem->setText("aaaaaa");
        listWidget->addItem(newItem);
    }

    listWidget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
    rightLayout->addWidget(listWidget);
}

MainWindow::~MainWindow()
{

}

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