When you try to show a QQuickWidget, which is a translucent (Qt::WA_TranslucentBackground) frameless tool window (Qt::FramelessWindowHint | Qt::Tool) on top of another window, it doesn't show up. Removing Qt::WA_TranslucentBackground attribute helps to show the window (not translucent though) and removing Qt::FramelessWindowHint helps to show the window (but with frames and not translucent either).

The attached example demonstrates the problem. The first button shows the QQuickWidget window and you can trigger the Qt::WA_TranslucentBackground on and off with the check box. The second button shows how it works if the QQuickWidget is a widget and not a window.

The problem is reproducible only on Windows (tested on Windows 7).