ListView creates and destroys delegates infinitely under certain conditions

You can see this with the same test as in https://bugreports.qt-project.org/browse/QTBUG-31203. The lack of an incubation controller is what makes it hang while it's doing that. But with or without the IC, if you click the button to increase the delegate height, flick down and then back up again, you will see the messages over and over about delegates at the same index being created and destroyed. More info by turning on DEBUG_DELEGATE_LIFECYCLE in qquicklistview.cpp.