QGraphicsScene::addWidget doesn't work when combined with an QGLWidget viewport with auto-swap buffers off

Steps to reproduce:
1. Download and open the example at http://doc.qt.digia.com/qq/qq26-openglcanvas.zip (from http://doc.qt.digia.com/qq/qq26-openglcanvas.html);
2. Make a few adjustments to compile with Qt 5;
3. Create a simple class derived from QGLWidget which does nothing but call setAutoBufferSwap(false);
4. In main.cpp, replace the QGLWidget usage with the custom class (these 2 steps are only necessary because setAutoBufferSwap is not public);
5. Add a QGLWidget or QGLContext field to OpenGLScene and call swapBuffers in drawBackground (this should've fixed the issue but it doesn't);
6. Run the example.

Expected results:
All widgets are rendered and react to input.

Actual results:
No widgets are rendered except the QGLWidget.