Win7: Frameless widgets have weird titlebar appears during few milliseconds when minimizing/restoring

Hi Qt experts,

I noticed a weird behavior when running a frameless widget in Qt. If I repeat the process of minimizing it in taskbar and show it multiple times, an old style title bar appears in the top left corner during a few milliseconds and then disappears.

Tested on Qt5.6.

You will find attached a very simple project where bug appears and a video showing the bug.