Prevent trying to parent items to themselves

objects/Grid.cs

@@ -34,6 +34,17 @@ public partial class Grid : Area3D
 	}
 
 
+	/// <summary> Returns whether the specified item is contained in this or any nested grids. </summary>
+	public bool ContainsItem(Item item)
+	{
+		while (item?.GetParentOrNull<Node>() is Grid grid) {
+			if (grid == this) return true;
+			item = grid.GetParent() as Item;
+		}
+		return false;
+	}
+
+
 	public Vector3I LocalToGrid(Vector3 pos)
 		=> (Vector3I)((pos + _halfGridActualSize) / StepSize);
 	public Vector3 GridToLocal(Vector3I pos)

player/PickupController.cs

@@ -88,7 +88,9 @@ public partial class PickupController : Node3D
 		EnsureCurrentItemValid();
 
 		if (HasItemsHeld) {
-			if ((RayToMouseCursor() is RayResult ray) && (ray.Collider is Grid grid)) {
+			if ((RayToMouseCursor() is RayResult ray) && (ray.Collider is Grid grid)
+					// Ensure item is not being added to itself or nested items.
+					&& !((grid.GetParent() == CurrentItem) || grid.ContainsItem(CurrentItem))) {
 				var transform = CurrentItem.GlobalTransform with { Origin = ray.Position };
 				transform = grid.Snap(transform, ray.Normal, CurrentItem);
 				_preview.GlobalTransform = transform;