SnackBar with action no longer auto-dismisses

The default behavior of a SnackBar with an action has changed. Previously, a SnackBar with an action would not auto-dismiss if talkback was enabled. Now, all SnackBars with an action default to a non-dismissible state until the user interacts with the action button.

A SnackBar with an action button is now treated as a more persistent notification that requires user interaction. This change improves accessibility and user experience by ensuring that critical notifications remain on the screen until they are acknowledged.

This change aligns with the Material 3 design specifications for SnackBars:

Old behavior: A SnackBar with an action button would auto-dismiss after a duration unless talkback was enabled.
New behavior: A SnackBar with an action button won't auto-dismiss; it remains on screen until dismissed by the user.

To override this behavior, an optional persist property has been added to SnackBar. When persist is true, the SnackBar won't auto-dismiss
and remains on screen until manually dismissed by the user. When false, the SnackBar auto-dismisses after its standard duration, regardless of the presence of an action. When null, the SnackBar follows the default behavior, which won't auto-dismiss if an action is present.

To restore the old auto-dismiss behavior for a SnackBar with an action, set persist to false.

Code before migration:

dartScaffoldMessenger.of(context).showSnackBar(
  SnackBar(
    content: const Text('This is a snackbar with an action.'),
    action: SnackBarAction(
      label: 'Action',
      onPressed: () {
        // Perform some action
      },
    ),
  ),
);

Code after migration:

dartScaffoldMessenger.of(context).showSnackBar(
  SnackBar(
    content: const Text('This is a snackbar with an action.'),
    persist: false, // Add this line to restore auto-dismiss behavior
    action: SnackBarAction(
      label: 'Action',
      onPressed: () {
        // Perform some action
      },
    ),
  ),
);