WDYT @mehmetf ?

I made some comments about this in the internal bug that Kate and I are discussion. Should we move the discussion here? I am particularly miffed about not being able to detect nested Scaffolds. It would be great to be able to throw an assertion error if we can detect a snackbar will appear multiple times on the same screen.

Sure! We can move discussion here. :)
The intention for handling nested Scaffolds with the ScaffoldMessenger is to place a ScaffoldMessenger in between them, Scaffolds will only receive SnackBars from the closest ancestor ScaffoldMessenger, so there would only be one SnackBar shown.

If you want to access a ScaffoldMessenger higher in the tree (above the closest ancestor), you can use a key. This follows the same pattern of accessing Scaffolds with keys to show SnackBars.

It might be possible to detect nested Scaffolds, I haven't thought of a way yet, but I'm not sure how we would then decide to handle the SnackBar. I don't know that we can assume which Scaffold the user would want to have their SnackBar in at that point.
This is why the ScaffoldMessenger defines a scope for showing SnackBars.

The API docs do have a troubleshooting section on nested Scaffolds here: https://api.flutter.dev/flutter/material/Scaffold-class.html I think we can definitely add to this.

I absolutely agree with everything you said 👍

If you place a ScaffoldMessenger between the two Scaffolds then each messenger will see exactly one Scaffold registered, correct?

What I meant was something like this. In ScaffoldMessengerState:

void _register(ScaffoldState scaffold) {
    assert(_scaffolds does not contain any item that is a parent of the scaffold, 'You should really scope your Scaffolds....');
    _scaffolds.add(scaffold);
    if (_snackBars.isNotEmpty) {
      scaffold._updateSnackBar();
    }
  }

If you place a ScaffoldMessenger between the two Scaffolds then each messenger will see exactly one Scaffold registered, correct?

Not necessarily, since there could be multiple Scaffolds registered with one ScaffoldMessenger from a stack of routes, or there could be more than one registered during a page transition, or across Tabs in a TabView, etc.

What I meant was something like this. In ScaffoldMessengerState:

void _register(ScaffoldState scaffold) {
    assert(_scaffolds does not contain any item that is a parent of the scaffold, 'You should really scope your Scaffolds....');
    _scaffolds.add(scaffold);
    if (_snackBars.isNotEmpty) {
      scaffold._updateSnackBar();
    }
  }

I see what you mean now, thank you! 🎉
I think maybe we can do this by checking if the Scaffold that is being registered has a Scaffold ancestor that is already registered with the ScaffoldMessenger.

There is also the case of side-by-side Scaffolds, which would result in multiple SnackBars as well. I don't know how we could detect that in the ScaffoldMessenger.

I am going to close this, I've filed #69929 for tracking these improvements. Thank you!