How to render context of QGLWidget, outside of the window

[KnufflPuffl]

Hey. I have written a QT application, which draws a lot of elments inside it. So I have made it possible to scroll inside this window.

Now I want to output ALL elements to a bitmap file. So far I am using the following code:

void GLOutput::renderPix(){
 
    int heighthelp = this->height();
    int helpwidth = this->width();
 
    this->resizeGL(helpwidth,2000); // my window has only a height of 800
 
    QPixmap qp = QPixmap::grabWidget(this,0,0,this->width(),this->height();
    qp.save(QFileDialog::getSaveFileName(this, tr("Save File"),"./untitled.png",tr("PNG File (*.png)")),"PNG");
 
    this->resizeGL(helpwidth,heighthelp); // resize it to the original size
}

void GLOutput::renderPix(){

    int heighthelp = this->height();
    int helpwidth = this->width();
    
    this->resizeGL(helpwidth,2000); // my window has only a height of 800
  
    QPixmap qp = QPixmap::grabWidget(this,0,0,this->width(),this->height();
    qp.save(QFileDialog::getSaveFileName(this, tr("Save File"),"./untitled.png",tr("PNG File (*.png)")),"PNG");

    this->resizeGL(helpwidth,heighthelp); // resize it to the original size
}

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The problem is that this code works fine for the actual context which is viewable for the user at the moment, but I need something that renders all elements inside the Widget. So my idea was simply to make the GLWidget bigger, before rendering, but this has no effect at all.
I think the problem is that all pixels which are not directly shown in the window are invalid.

Any ideas are very appreciated. I simply don't know how to define the renderArea I want to grab.

thx in advance