Linux, when published as snap: Closing one window terminates application

When an app using desktop_multi_window is published to the Linux snap store, closing one window closes the entire app. When the same app is run in the development environment, this does not happen. Perhaps there's some kind of GTk-level registratino that's off?

Reproduce Steps

You can see the behavior with, for example, https://snapcraft.io/jrpn15/ . If you install the snap on LInux, run it, and do File -> See Calculator Internals, it opens a sub-window. Close that sub window using the native window manager, and the entire app terminates.

I tried closing the window programmatically, using WindowController.close() to close the window, and the same thing happens: Both windows close, and the app terminates.

When I do the same thing with a non-snap version of the same program, just the one window closes.

**Version **

• Flutter Version: 3/3/8
• OS: Linux (tested on Ubuntu)
• plugin: desktop_multi_window

Reproduce the failure by installing the jrpn15 snap on Ubuntu, opening File → See Calculator Internals, and closing the sub-window natively or through WindowController.close(). Start by examining the desktop_multi_window Linux window lifecycle and its GTK/snap behavior. Done means closing the child window leaves the main application running, as it does outside the snap.