Expand the widgets inside a scrollArea to the maximum possible size, or the window to the minimum possible size.

FullWindowWidthScrollArea.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QVBoxLayout>
#include <QScrollArea>
#include <QSlider>
#include <QWidget>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QVBoxLayout *mainLayout = new QVBoxLayout();

    // Add sliders to the layout so it overflows the window height for sure
    for(int i = 0; i < 100; i++) {
        QSlider *slider = new QSlider(Qt::Horizontal);
        mainLayout->addWidget(slider);
    }

    QWidget *mainWidget = new QWidget();
    mainWidget->setLayout(mainLayout);

    QScrollArea *scrollArea  = new QScrollArea();
    scrollArea->setWidgetResizable(true);
    scrollArea->setWidget(mainWidget);

    setCentralWidget(scrollArea);
}

MainWindow::~MainWindow()
{
    delete ui;
}