Windows: QComboBox menu cannot stay expanded when used in a dock widget that has been redocked

1. Undock the the dockwidget from the main window. Do NOT click on the QComboBox before this.
2. Click on the QComboBox to get the menu.
3. Redock the QDockWidget.
4. Click on the QComboBox and notice now that the menu will appear but then immediately collapse.

You will get also warning:
QWindowsWindow::setMouseGrabEnabled: Not setting mouse grab for invisible window QWidgetWindow/'QComboBoxPrivateContainerClassWindow'
QWindowsBackingStore::flush: GetDC failed (Invalid window handle.)

Below is code which reproduces this problem. Does not occur in Qt4.8.5.

#include <QApplication>
#include <QMainWindow>
#include <QComboBox>
#include <QDockWidget>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;
    QDockWidget * dock2 = new QDockWidget(&w);
    QComboBox * comboBox = new QComboBox(dock2);
    comboBox->addItem("text1");
    dock2->setWidget(comboBox);

    w.addDockWidget(Qt::RightDockWidgetArea, dock2, Qt::Vertical);
    w.show();

    return a.exec();
}