QWindow with OpenGL alpha blending becomes translucent

#include <QApplication>

#include <QWindow>
#include <QOpenGLContext>
#include <QTimer>
#include <QPointer>
#include <QOpenGLFunctions_4_1_Compatibility>


class CRenderTimer : public QTimer
{
    QPointer<QOpenGLContext> ctx;
    QPointer<QWindow> wnd;
    QOpenGLFunctions_4_1_Compatibility *GL;
public:
    CRenderTimer(QPointer<QOpenGLContext> context, QPointer<QWindow> window)
        : QTimer(), ctx(context), wnd(window)
    {
        if(ctx->makeCurrent(wnd))
        {
            GL = ctx->versionFunctions<QOpenGLFunctions_4_1_Compatibility>();
            GL->initializeOpenGLFunctions();

            GL->glDisable(GL_DEPTH_TEST);
            GL->glEnable(GL_BLEND);
            GL->glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
            GL->glClearColor(1,1,1,1);

            ctx->doneCurrent();
        }
    }
    ~CRenderTimer()
    {

    }
    protected:
    void timerEvent(QTimerEvent *)
    {
        if(ctx->makeCurrent(wnd))
        {
            GL->glDrawBuffer(GL_BACK);
            GL->glClear(GL_COLOR_BUFFER_BIT);

            GL->glViewport(0,0,wnd->width(), wnd->height());

            glMatrixMode(GL_PROJECTION);
            glLoadIdentity();
            //GL->glOrtho(0,wnd->width(), 0, wnd->height(), -1, 1);
            glMatrixMode(GL_MODELVIEW);
            glLoadIdentity();


            glBegin(GL_QUADS);
            glColor4f(1,0,0,1);
            glVertex3i (-1, -1, -1);
            glVertex3i (1, -1, -1);
            glVertex3i (1, 1, -1);
            glVertex3i (-1, 1, -1);

            glColor4f(0,0,1,0.5);
            glVertex3f (-0.5, -0.5, -1);
            glVertex3f (0.5, -0.5, -1);
            glVertex3f (0.5, 0.5, -1);
            glVertex3f (-0.5, 0.5, -1);

            glEnd();
            //ctx->doneCurrent();
            ctx->swapBuffers(wnd);
        }
    }

};



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

    QSurfaceFormat glFormat;
    glFormat.setVersion(4,1);
    glFormat.setProfile(QSurfaceFormat::CompatibilityProfile);
    QPointer<QOpenGLContext> context(new QOpenGLContext());
    context->setFormat(glFormat);
    context->create();

    QPointer<QWindow> wnd(new QWindow);
    wnd->setFormat(context->format());//Calling wnd->setFormat(glFormat) will solve the issue
    wnd->setSurfaceType(QSurface::OpenGLSurface);
    wnd->create();

    wnd->show();

    CRenderTimer * renderTimer = new CRenderTimer(context, wnd);
    renderTimer->setInterval(20);
    renderTimer->start();


    return a.exec();

}