#include <QApplication>
#include <QTreeView>
#include <QStandardItemModel>
#include <QHeaderView>
#include <QTimer>

class TableModel : public QStandardItemModel
{
    int size;
public:
    TableModel() : size(0) {}

    void update()
    {
        ++size;
        clear();
        setColumnCount(size);
        for (int x = 0; x < size; ++x)
            setHeaderData(x, Qt::Horizontal, QString::fromLatin1("column %1").arg(x));
    }
};

class TreeView : public QTreeView
{
    Q_OBJECT

public:
    QTimer t;

    void update()
    {
        static_cast<TableModel *>(model())->update();
        resizeColumnToContents(0);
//        header()->setStretchLastSection(true); <--- fixes it
    }

    TreeView(QWidget *parent = 0) : QTreeView(parent)
    {
        setGeometry(0,0,500,100);
        TableModel *m = new TableModel();
        t.setInterval(1000);
        connect(&t, &QTimer::timeout, this, &TreeView::update);
        setModel(m);
        header()->setStretchLastSection(true);
        t.start();
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    TreeView t;
    t.show();
    return a.exec();
}

#include "main.moc"