Qt menubar disappears when I use scroll area

[high_flyer]

You are placing your central widget in to the scorll area, instead parenting to it.

QScrollArea *p_ScrollArea = new QScrollArea(centralWidget());
    p_ScrollArea->setAutoFillBackground(false);
    centralWidget()->setGeometry(0,0,500,500);
    p_ScrollArea->setGeometry(0,0,100,100);
    QLabel* pLabel = new QLabel("test");
    pLabel->setGeometry(0,0,1000,1000);
    p_ScrollArea->setWidget(pLabel);

QScrollArea *p_ScrollArea = new QScrollArea(centralWidget());
    p_ScrollArea->setAutoFillBackground(false);
    centralWidget()->setGeometry(0,0,500,500);
    p_ScrollArea->setGeometry(0,0,100,100);
    QLabel* pLabel = new QLabel("test");
    pLabel->setGeometry(0,0,1000,1000);
    p_ScrollArea->setWidget(pLabel);

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Also, setting the minimum size will not do when you change the size of your window.
You need to make sure your scoll areay adjust to the windows size.