See attached self contained example.
Subclassing QQuickView::event and returning false on an event of type QEvent::Close doesn't prevent the application to quit.
This mechanism works on Linux and Windows but not on macOS (I'm using 10.13.4)
So basically I have no way to implement a "Save before exit" step.
[EDIT] after comments (and adding event->ignore()), the issue is now only related to Cmd+Q or "Quit" from menu. I haven't found a way on macOS to prevent the application to quit
|For Gerrit Dashboard: QTBUG-72013|
|277640,13||Deliver Quit event when calling QCoreApplication::quit()||dev||qt/qtbase||Status: MERGED||+2||0|
|277851,1||Teach QCoreApplication::quit() to involve platform in termination||5.14||qt/qtbase||Status: ABANDONED||0||0|
|312898,3||Plumb application quit through platform plugin||dev||qt/qtbase||Status: MERGED||+2||0|