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.