QFontDialog: currentFont() doesn't work

currentFont() is not updated correctly when the user changes the font in QFontDialog.

Reproduction:
1. start the attached program on Mac
2. change the font, e.g. to "Tahoma"
3. see the debug log:
"currentFont() should be: Tahoma ..."
"but is: Arial ..."

I can reproduce this with Qt 5.9.3 and Qt 5.9.6 on macOS 10.13.5, but NOT on Win. With Qt 5.6.2, it is all right on Mac.

The missing update seems to occur only when the QFontDialog constructor with initial font is used. When the empty constructor QFontDialog() is used (see alternative code in the example), it works as expected.