QTabBar::setExpanding(true) is ignored in some cases

#include <QApplication>
#include <QTabWidget>
#include <QTabBar>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QTabWidget tabWidget;
    for (int i = 0; i < 3; i++) {
        tabWidget.insertTab(i, new QWidget(), QString("Tab %1").arg(QString::number(i)));
    }
    tabWidget.setTabPosition(QTabWidget::North);
    tabWidget.setDocumentMode(true);
    tabWidget.tabBar()->setExpanding(true);
    tabWidget.resize(640, 480);
    tabWidget.show();

    return app.exec();
}