Flutter Dismissible insists list item must be removed from tree

You can use confirmDismiss function of Dismissible widget for this purpose.

If you don't want the widget to get dismissed, then you just need to return false from confirmDismiss.

Don't use onDismissed to do your post-swipe processing, use confirmDismiss instead, it will provide you with the swipe direction just as onDismissed.

Here is the official documentation for confirmDismiss function:

Gives the app an opportunity to confirm or veto a pending dismissal. If the returned Future completes true, then this widget will be dismissed, otherwise it will be moved back to its original location. If the returned Future completes to false or null the [onResize]

and here is an example:

Dismissible(
  confirmDismiss: (direction) async {
    if (direction == DismissDirection.startToEnd) {
      /// edit item
      return false;
    } else if (direction == DismissDirection.endToStart) {
      /// delete
      return true;
    }
  },
  key: Key(item.key),
  child: Text(item.name),
)

The Dismissible will think your item was dismissed as long as the item key changes. Let's say your item class is MyItem. If you implement a constructor MyItem.from in your MyItem class that copies the fields over, e.g.:

class MyItem {
  MyItem({ @required this.title, @required this.color });
  MyItem.from(MyItem other) : title = other.title, color = other.color;
  final String title;
  final Color color;
}

Then you can replace handleUndo(item) with handleUndo(new MyItem.from(item)) so that your new ObjectKey(item) will be unique from the old ObjectKey that you used before (assuming you didn't implement operator == on MyItem).