dragging the tab will cause it to be selected

#include <QApplication>
#include <QMainWindow>
#include <QTabWidget>
#include <QVBoxLayout>

class TestWindow : public QMainWindow
{
public:
    TestWindow(QWidget *parent = nullptr) : QMainWindow(parent)
    {
        QTabWidget* tw = new QTabWidget;

        QWidget* w1 = new QWidget;
        QWidget* w2 = new QWidget;

        tw->addTab( w1, "One" );
        tw->addTab( w2, "Two");
        tw->setMovable(true);

        QVBoxLayout* l = new QVBoxLayout;
        l->addWidget(tw);

        QWidget* w = new QWidget;
        w->setLayout(l);

        setCentralWidget(w);
    }
};

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