[OS X 10.13] Minimized windows don't die

Make a window. Minimize the window. You now see the window icon in the dock.
Now kill the window. It is still in the dock.
Click the window's icon in the dock, the window normalizes.
Now you can kill the window.

In a bit more detail, if you make a window and connect to QObject::destroyed, the signal is delivered when the widget is destroyed. But the icon remains in the dock. I have traced into ~QWidget and on down to ~QCocoaWindow, where there is a call to [nswindow release]. But the window simply doesn't completely die.

This happens on OS X 10.13.3. It does not happen on 10.12.6. I have tested Qt 5.6.3 and 5.9.3; both have the problem.

I attach a small application. My apologies- it is a zipped project with multiple files, but easy to build. Build and run- it makes a window with two buttons. Click the Make Windows button, two windows appear and slide into the dock. Click the Kill Them button, a message from qDebug() appears in the console telling you that the destroyed signal was received. But the windows are still present in the dock.

We discovered this via a customer bug report.