The new API QFileSystemModel::directoryLoaded as a bad side affect. To demonstrate it, run the Dialog sample and open the file selector (not native) and select a desired path. Now from Windows Explorer, save a file in the watched folder.
...This will cause the QCompleter to popup.

In my application, I have various fields with QFileSystemModel / QCompleter, not in modal dialogs. And from time to time, the completer popups "without reasons" (from the user point of view).

I think the directoryLoaded signal should not cause this effect.