Strange behavior of QColorDialog, when called from QStyledItemDelegate subclass

Running the attached C++ example shows a strange behavior on KDE desktops: double clicking on an item opens a QColorDialog. Attempting to move the dialog before selecting a color makes the dialog disappear with Qt 5.8.0, while older versions (tested with 5.5.1) doesn't show this behavior. Further examinations indicate, that other platforms might not be affected.

This is the simplest C++ version, I can come up with. It is based on PyQt scripts, that I've published on stackoverflow:
http://stackoverflow.com/questions/43298605/strange-behavior-of-qcolordialog-when-called-from-qstyleditemdelegate-subclass

This (related, slightly more complex) script even segfaults after closing the QColorDialog:
http://stackoverflow.com/questions/43299616/calling-qcolordialog-triggered-from-qstyleditemdelegate-editor-results-in-crash

Inquiry on PyQt ML:
https://www.riverbankcomputing.com/pipermail/pyqt/2017-April/039059.html