Reproducer attached. One needs to build "Child" first, winqtdeployqt it, and then run "Parent". It is going to launch "Child" and then embed the window in itself.

"Child" has a button which blocks the main thread by clicking. So when "Child" is blocked, "Parent" is unresponsive too.

I've tested both 6.7 and 6.8, starting "Child" detached or non-detached. I've also tried to embed "Child" into a QML Window (https://www.qt.io/blog/window-embedding-in-qt-quick). Nevertheless, none of those makes any difference. And it is said that the problem does not happen on Linux. So I tend to think it's a Windows issue but can't be sure.