X to close button disabled for QFontDialog (and others)

If I build the standard dialogs example with Qt 5.1 and Visual C++ 2012, the X button in the upper right hand corner is disabled for QFontDialog. It is enabled for QColorDialog and others in the same example. I first noticed this when all the dialogs in my code had the same problem. I found I could fix it by adding this->setWindowFlags(Qt::Window); But it's not obvious how to fix it for QFontDialog. (There is a cancel button unless you turn it off, so it's not a big problem.) Qt 4.84 did not behave this way, Qt 5.02 does. I don't have all possible compiler versions installed, but I don't see any compiler or 32/64-bit differences. I don't see anything in the docs indicating whether the old or new behavior is expected.