QScrollArea, QPalette

[spyral]

Hello, everyone.
I'm new to Qt-programming and have the following problem:

MyClassConstructor::MyClassConstructor( QWidget* parent )
    : QWidget( parent )
{
        QScrollArea* area = new QScrollArea;
        QWidget* w = new QWidget;
 
        QVBoxLayout* layout = new QVBoxLayout;
        layout->addWidget( area );
 
        w->resize( 500, 500 );
        QPalette p( m_table->palette() );
        p->setColor( QPalette::Window, Qt::red );
 
        area->setWidget( w );
        area->viewport()->setAutoFillBackground( true );
        // area->viewport()->setStyleSheet( "QWidget{background:green}" );
        area->viewport()->setPalette( p );
 
        this->setLayout( layout );
}

MyClassConstructor::MyClassConstructor( QWidget* parent )
    : QWidget( parent )
{
        QScrollArea* area = new QScrollArea;
        QWidget* w = new QWidget;

        QVBoxLayout* layout = new QVBoxLayout;
        layout->addWidget( area );

        w->resize( 500, 500 );
        QPalette p( m_table->palette() );
        p->setColor( QPalette::Window, Qt::red );

        area->setWidget( w );
        area->viewport()->setAutoFillBackground( true );
        // area->viewport()->setStyleSheet( "QWidget{background:green}" );
        area->viewport()->setPalette( p );

        this->setLayout( layout );
}

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If I uncomment the line with setStyleSheet, the widget is wonderful red. But using the QPalette won't give a result.
What is thing, I do not understand?
I'm new here, so I hope I respect the rules of the forum.
Thank you.