REG->5.13.1/Windows 7:Close button does not work on non-resizable apps with Areo theme disabled

Hi, using Qt 5.13.1 on Windows 7 classic or basic desktop (i.e. with Aero disabled, non-transparent menu bars) the close button (the red X) no longer works if I set the app's minimum and maximum sizes to be the same.

On Windows 10 or Windows 7 with Aero enabled the red close button works. On earlier Qt versions, like Qt 5.12.x or 5.7.x, the close button also works fine on Windows 7 classic or basic desktop. This behavior seems to have started with Qt 5.13 (haven't tested 5.13.0).

Steps to reproduce:

Using Qt Creator, create an empty vanilla widget application. In MainWindow's constructor, insert these 2 lines after ui->setupUi(this); :

setMinimumSize(800,800);
setMaximumSize(800,800);

Currently I use the workaround setMinimumSize(800,801); to get the red close button to work again in Qt 5.13.1 but the problem is that the mouse cursor changes to indicate that a vertical resize is possible