single buffer OpenGL does no longer work on OSX

Enabling single buffered drawing for a QGLWidget does no longer work.
It worked in Qt 4.8.x.
If a QGLWidget is created with a QGLFormat with "setDoubleBuffer(false)" then the resulting QGLWidget is still double buffered and flushing drawing with glFlush() or glFinish() does draw nothing.
In the attached example the GLWidget creator will abort if the demo was compiled with 5.0.0.