ISnackbar.ShownSnackbars could expose a deferred enumeration over the mutable backing list after the lock had been released. This change narrows the fix to that concurrency edge by returning a stable snapshot and adding coverage for enumeration after subsequent mutations.

• Snackbar service

  • Return a materialized snapshot from ShownSnackbars instead of a deferred Take(...) over the live list.
  • Preserve existing MaxDisplayedSnackbars behavior and ordering while making callers safe from post-read mutations.

• Regression coverage

  • Add a service-level test that captures ShownSnackbars, mutates the service state, and verifies the original enumeration remains unchanged.

public IEnumerable<Snackbar> ShownSnackbars
{
    get
    {
        _snackBarLock.EnterReadLock();
        try
        {
            return _snackBarList.Take(Configuration.MaxDisplayedSnackbars).ToArray();
        }
        finally
        {
            _snackBarLock.ExitReadLock();
        }
    }
}