QQuickWidget in a QDockWidget causes severe resize issues

If using QQuickWidgets inside DockWidgets, on resize, the mainWindow's menu flickers / disappears, and sometimes is still moved outside of the window after mouseUp after the resizing. If the menu is outside of the window, there is also a black area at the bottom of the mainWindow. (This black area is only visible directly, not via screenshare by the way.)

I have only tested on Windows, and it only happens with the default Direct3D renderInterface. Setting QQuickWindow::setGraphicsApi(QSGRendererInterface::OpenGL or Software) works fine as a workaround.

The same code works fine within Qt 5.15.2. 

Within Qt 6.3.1, it won't run and yields a message "QQuickWidget is only supported on OpenGL. Use QQuickWindow::setGraphicsApi() to override the default."

I have attached the whole test project as zip, and additionally the mainwindow.cpp itself for quick reference.