macOS: Problem of QtQuick FileDialog from Qt.labs.platform

This is some kind of follow-up bug to QTBUG-76523, where Ulf Hermann suggested to use the QtQuicks FileDialog from Qt.labs.platform instead of the Quick Control 1 FileDialog.

We reproduced a part of the problem in the attached QtCreator project started from a new, empty Qt Quick application.

In main.qml, we added the FileDialog with a couple of parameter options for modality and fileMode. Depending on the set parameters a file dialog opens or does not open (nothing happens).

Our observation is:

• modality: Qt.NonModal; fileMode: FileDialog.OpenFile; // This does open a filedialog
• modality: Qt.WindowModal; fileMode: FileDialog.OpenFile; // This does open a filedialog
• modality: Qt.ApplicationModal; fileMode: FileDialog.OpenFile; // This does NOT open a filedoalog
• modality: Qt.NonModal; fileMode: FileDialog.SaveFile; // This does NOT open a filedoalog
• modality: Qt.WindowModal; fileMode: FileDialog.SaveFile; // This does open a filedoalog
• modality: Qt.ApplicationModal; fileMode: FileDialog.SaveFile; // This does NOT open a filedoalog

The context, where we observe the file dialog problem is in a VST audio plugin, done with QtQuick. The only situation that opens a file dialog here is for modality=Qt.NonModal and fileMode=FileDialog.OpenFile. In the end, this means that no save dialog at all can be opened on OSX.

The different behavior compared to the attached project is probably due to the fact that we embed our plugin UI into a NSView generated by the host application.