QGLWidget draws wrong within QMainWindow on Mac OS

If a QGLWidget is placed within a QMainWindow, it draws over the entire window rather than just within its own bounds.

I've included a trivial application which adds a button on the left and a QGLWidget on the right.

Under 4.8, the button is visible and the background of the window draws correctly.

Under 5.0 (Mac OS), the background of the window is black and the button is not visible.