Option for tscn files to have a designated child destination node

[yahkr]

While attempting to create a generic Window for my game's ui I ran into this puzzle. If I have the following scene:

and add a button to it this is what happens:

This is what I would expect to happen:

And this works with editable children, however if I now modify the scene file in some way that moves the ContentContainer child, all hell breaks loose and the button is no longer a child of the ContentContainer and becomes named its old location and is not deletable until I restart the entire editor.

---

This is how I would hope it would work. Being able to add/remove children without worry about the underlying scene changing, all they care about is where do I end up when added as a child.

I did attempt to create a custom node with the following basic code, but with this solution the children are inaccessible.

[Tool]
public partial class DraggableWindow : VBoxContainer
{
    private Node root;
    private Node contentContainer;
    
    public override void _Ready()
    {
        var scene = GD.Load<PackedScene>("res://addons/JUI/Window.tscn");
        root = scene.Instantiate();
        contentContainer = root.GetNode<VBoxContainer>("%ContentContainer");
        AddChild(root);
    }
    public override void _Notification(int what)
    {
        if (what == NotificationChildOrderChanged)
        {
            if(GetChildren().Count > 1)
            {
                GetChild<Node>(1).Reparent(contentContainer);
            }
        }
    }
}

[theraot]

See #8184