[Reg 5.11.1->5.13.1]Minimize and close buttons not working for a fixed size window with Windows 7

On Windows 7, of the size of window is set as fixed, minimize and close buttons stop working. Just add following line in any default sample code,

this->setFixedSize(width(), height());

If the Window does not have the focus and another window is in front and overlapping (for example the explorer), you are able to use the buttons again!

Please refer to the attached sample and run it with Qt 5.13.1 or 5.12.5 on Windows 7 machine.