Windows: Desktop OpenGL: render() not called when window shrinking in size

The version is 5.1.0 RC2, but that can't be selected from the Affects Version/s drop down list!

This short test program uses the openglwindow.h in examples/gui/openglwindow/openglwindow.h:

#include <QtGui>
#include "openglwindow.h"
#include <QPainter>
#include <iostream>

class OpenGLFail : public OpenGLWindow
{
public:
OpenGLFail();

protected:
void render(QPainter *p);
};

OpenGLFail::OpenGLFail()
{
setTitle("OpenGL Test");
resize(500, 800);
}

void OpenGLFail::render(QPainter *p)
{
std::cout << "width(): " << width()
<< " height(): " << height() << endl;
std::cout.flush();
}

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

OpenGLFail test_window;

test_window.show();

app.exec();
}

As can be seen in the Application Output screen in QtCreator, the width and height do not display as the test window is reduced in size. This test program works fine in Qt 5.0.2