Big flickering on frameless Window resizing (regression on OSX)

See attached sample.
Run it. Resize the window from any border (make the window big enough). Observe the extreme flickering, caused by the window being transparent a short time at each resizing event... amateurish feeling
This problem is caused by this change: https://codereview.qt-project.org/#/c/108362
Revert this change (remove the 2 lines) and the problem is completely solved.

I use a QDockWidget for the sake of simplicity, to be able to resize the window easily. But this is not a QDockWidget problem and the same happens with any frameless widget (eg. menu)

Related story https://bugreports.qt.io/browse/QTBUG-47354

I did not try 5.6, but the code is the same, hence I guess 5.6 is also affected.