How to show FBO contents in a QGLWidget?

[Justin Sayne]

I basically tried it this way:

  class Foo : public QGLWidget
  {
  public:
    Foo( QWidget* parent ) : QGLWidget( parent )
    {
    }

    void paintGL() override
    {
      QOpenGLFramebufferObject  fbo( size( ) );
      fbo.bind( );
      glClearColor( 1.0f, 0.0f, 0.0f, 1.0f );
      glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT );
      fbo.release( );
      bindTexture( fbo.toImage( ), fbo.format( ).textureTarget( ), fbo.format( ).internalTextureFormat( ) );
      drawTexture( rect( ), fbo.texture( ), fbo.format( ).textureTarget( ) );
    }
  };

but the widget will just show a black background then. I've also tried to bind the fbo texture and draw a quad - same result. If I uncomment the fbo code and draw directly to the GLWidget it works as expected. Any hints about what I'm doing wrong here?