I've just tested out the QQuickWidget class and it's working as I hoped. I can give each QQuickWidget it's own separate QMLEngine instance and the qml items inside each widget are not able to reach out into Qt widget land. There are still a few issues I'm looking into though regarding displaying one QQuickWidget over another. I've worked out how to make the top widget transparent but I'm still looking into event handling and how to stop the top QQuickWidget from eating events that should really be processed by the bottom QQuickWidget. I'm sure it's possible though. It's a shame that this isn't possible from QML itself. It'd be lovely to have a option you just pass to a Loader to say to sandbox the loaded item or something along those lines but this will work for now.

@romsharkov Hi there, Do I understand correctly that you have managed to run a QQmlProcess in a separate process? I am asking it because I have came across pretty much the same problem: https://github.com/sailfish-sdk/sdk-harbour-rpmvalidator/pull/79#issuecomment-307348648 If you could tell me some words of you implementation details that would be awesome! Thanks in advance!

@ganeshkbhat Allowing to load and run 3rd C++ inside your main applications's process is very dangerous and we have not found any easy way to sandbox C++ so far, only just have a few ideas worth trying. I've described this issue here in a little more detail.