When QGLWidget is added as a second widget into QSplitter, a strange white rectangle in the left upper corner of MainWindow appears. It is not only a visual error, but also the cursor is changed over this rectangle. The size of the rectangle is independent on the size of widgets or the main window.

The bug can be triggered by the following code:

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{
    QSplitter *splitter = new QSplitter();

    splitter->addWidget(new QLabel("Test"));
    splitter->addWidget(new QGLWidget());

    setCentralWidget(splitter);
}