QDockWidget lost it contents when QGLWidget is used at central widget of main window

QDockWidget content is not visible when QMainWindow has QGLWidget set as centralWidget. Demo application which can reproduce this issue can be used from QTBUG-28872.

Steps to reproduce:

• launch application
• undock dockwidget

Expected behaviour:

• dockwidget content should be visible

Actual behaviour:

• dockwidget content is not shown