QOpenGLWidget doesn't work when placed in a QDockWidget with QMainWindow::GroupedDragging enabled

I am unable to get window composition to function correctly when using a QOpenGLWidget in conjunction with QDockWIdgets, when QMainWindow::GroupedDragging is enabled, and winId() has been called on one of the dock widgets.  The QOpenGLWidget paintGL() method is working, the framebuffer gets painted correctly, but the composeAndFlush() call in QWidgetBackingStore fails.  I get the error:
composeAndFlush: makeCurrent() failed

To reproduce:

1. Build and run the attached application
2. Drag the dock widget in the main window on top of the floating one
3. The dock widgets become grouped
4. Switch Tabs to Dock Widget 2
5. Resize the floating window.  The window is not repainted.