If you turn off delayed sync, the app will start to sync as soon as the user finished the form and clicks save.
This won’t prevent the user from quickly closing the tab after clicking save, but it will present them with the “Saving Changes” and “Syncing the App” progress bars. That should make it clear to most users that the changes are still uploading, and keep them from closing it too quickly.
Unfortunately, this is an app-wide setting that can get pretty annoying in other parts of your app, so it may cause more problems than it’s worth.
You could also experiment with a Form-Save action that forces a sync. I’m not sure if that would work, but it’s worth a try.
See last line for forced sync: