Steps to reproduce:

Create QMainWindow and set QMainWindow::GroupedDragging option.
Create two docks: A and B. Take them out of main window and tab together.
Switch to tab B. The window title changes to B (correct).
Press the windows's "X" button to close tab B.
The window title of B remains , although A tab is the current one and tab B is closed.
The expected behavior is that the title changes to A when B is closed.

Example code:

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

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

    QMainWindow w;
    w.setDockOptions(QMainWindow::GroupedDragging);
    w.addDockWidget(Qt::LeftDockWidgetArea, new QDockWidget("A"));
    w.addDockWidget(Qt::RightDockWidgetArea, new QDockWidget("B"));
    w.show();

    return a.exec();
}