If you call a nonmodal QWindow from an applicationmodal QDialog, the QWindow appears behind the QDialog and is inoperable.
To demonstrate this issue, I extended the qt-example 'windowflags' (see attached zip-File).
From the 'windowflags' - Window, choose 'QDialog' and 'applicationmodal', click 'show Window' (window.png)
From the 'Preview' - Window, choose 'Window' and 'nicht modal', click 'show Window' (dialog_applicationmodal.png)
The next Window shows the resulting error (window_unreachable.png).

Same code works under MS-Windows 8.1 as expected (non-modal QWindow in front of an application-modal QDialog).