QGLWidgets invisible (transparent)

Hi,

with Qt 5.12.0 RC, QGLWidgets appear completely transparent in our app, i.e. the desktop is shown instead of the contents of the widget. The QGLWidgets are located inside QDockWidgets. If undocked, the areas of the QGLWidgets appear black. All this still worked fine with Qt 5.12 beta2.

Note that in the screenshot, three areas of the window are actually transparent.