How does this interact with the save confirmation dialogs?

@carrotIndustries Tested again just to be sure:

• No unsaved changes: App will quit immediately
• Unsaved changes, only editor open: "Save changes" dialog opens, app will not quit
• Unsaved changes, "Save changes" dialog is already open: A second (or third, or fourth…) "Save changes" dialog is opened, app will still not quit

I don't think having multiple dialogs is what the user expects, but I can't think of an easy way to ensure only a single dialog instance (at least at this time of day).

Also, I never managed to open a second standalone editor window on macOS, so I don't know what happens if multiple windows are open. Probably just one "Save changes" dialog for each window though.

Dune 3D not being single-instance on mac os brings some problems:

• The list of recent documents gets written to disk when the app closes, so when there are multiple instances, the last ones wins
• Preferences get saved on closing the preferences window. Also, preferences won't propagate to other instances, so the preferences window will show the settings from when the instance started.

How's it with other apps such as gtk4-demo-application?

I'm not sure what you mean here - Dune 3D is single-instance on macOS after all, but only when running as a packaged app: The macOS launch daemon takes care of enforcing the single-instance constraint, but it's not active when running the raw binary from the command line.

For all intents and purposes, we can consider Dune 3D a single-instance application when distributed as an app bundle (which should ideally be the default way).

As for the multiple "Save changes" popups, I think a static class member that indicates the currently active popup (either a pointer or a flag) should do the trick.

Tried it myself and implementing the 'quit' action should be preceded by implementing a single confirm to close dialog as we have it in Horizon EDA:

Right now, we get multiple dialogs all at once with only one being usable.

FYI, I'm off-and-on implementing such a dialog in Dune 3D since I wanted it sooner or later anyhow and this is a good excuse for doing so.