If garbage collection is lazy enough, it feels like a BUG, when it does its job. In this case the Qt/QML/JavaScript garbage collector was garbage collecting the dynamically created window some minutes or hours later as I used the program. It would just vanish. No more problems once I held a reference in an array to it, but if the garbage collector didn't take so long to reclaim it, I would have figured out my mistake when I first started using it rather than weeks later.