In the sample code, a child window(QMainWindow) is opened by clicking the toolbar button in the main window (also QMainWindow), if repeating the child window opening and closing for several times. The application will freeze. Inside both the child and main window, QQuickWidget is used with Qt3D Qml running in it.
The symptom has been observed in 5.12. It is NOT happening in 5.15. It is not perfect in 5.14 either, but the symptom is lighter (the application will not freeze, but you need to move the mouse a little bit for the rendered image in QQuickWidget to display ).
In the sample, the line "d->setAttribute(Qt::WA_DeleteOnClose);" has been commented out. Uncomment this line can make the child window and widgets in it get deleted when the child window is closed. Under this condition the symptom still happens, but it happens after more times of repeating the opening and closing of the child window.
The issue looks to be related to 3D rendering. When OpenGL is used instead of OpenGLES(Angle), the issue does not happen.