Godot 4 Node extension for getting Node2D bounding box based on descendents.

Node2DExtension.cs

using System.Linq;

public static partial class Node2DExtension
{
	public static Rect2 GetCalculatedBounds(this Node2D node2D)
	{
		var rect = new Rect2();
		var children = node2D.GetChildren().ToList();

		while (children.Count > 0)
		{
			Node child = children[^1];
			children.RemoveAt(children.Count - 1);
			children.AddRange(child.GetChildren());

			if (child is not Node2D node2DChild)
				continue;

			// Check for Godot get_rect or C# custom GetRect method
			var rectCallback = node2DChild.HasMethod("get_rect") ? "get_rect" :
				node2DChild.HasMethod("GetRect") ? "GetRect" : null;

			if (string.IsNullOrEmpty(rectCallback))
				continue;

			var childRect = (Rect2)node2DChild.Call(rectCallback);
			childRect.Position = node2D.ToLocal(node2DChild.ToGlobal(childRect.Position));
			rect = rect.Merge(childRect);
		}

		return rect;
	}
}