disallow grouping and removing group i/o nodes

classes/deck/deck.gd

@@ -143,11 +143,14 @@ func is_empty() -> bool:
 
 
 ## Remove a node from this deck.
-func remove_node(uuid: String, remove_connections: bool = false, keep_group_instances: bool = false) -> void:
+func remove_node(uuid: String, remove_connections: bool = false, force: bool = false, keep_group_instances: bool = false) -> void:
 	var node := get_node(uuid)
 	if node == null:
 		return
 
+	if !node.user_can_delete && !force:
+		return
+
 	if node.node_type == "group_node" && !keep_group_instances:
 		DeckHolder.close_group_instance(node.group_id, node.group_instance_id)
 
@@ -179,7 +182,13 @@ func remove_node(uuid: String, remove_connections: bool = false, keep_group_inst
 func group_nodes(nodes_to_group: Array) -> Deck:
 	if nodes_to_group.is_empty():
 		return null
-
+		
+	# don't include nodes that can't be grouped/deleted
+	nodes_to_group = nodes_to_group.filter(
+		func(x: DeckNode):
+			return x.user_can_delete
+	)
+	
 	var node_ids_to_keep := nodes_to_group.map(
 		func(x: DeckNode):
 			return x._id

classes/deck/deck_node.gd

@@ -41,6 +41,10 @@ var props_to_serialize: Array[StringName]
 ## Only used by renderers.
 var position: Dictionary = {"x": 0.0, "y": 0.0}
 
+## If [code]true[/code], the user can delete this node by normal means.
+## The parent [Deck] can still delete the node by other means.
+var user_can_delete: bool = true
+
 enum PortType{
 	INPUT, ## Input port type (slot on the left).
 	OUTPUT, ## Output port type (slot on the right).

classes/deck/nodes/group_input_node.gd

@@ -12,6 +12,7 @@ func _init() -> void:
 	node_type = "group_input"
 	props_to_serialize = [&"output_count"]
 	appears_in_search = false
+	user_can_delete = false
 
 	add_output_port(
 		DeckType.Types.ANY,

classes/deck/nodes/group_output_node.gd

@@ -12,6 +12,7 @@ func _init() -> void:
 	node_type = "group_output"
 	props_to_serialize = [&"input_count"]
 	appears_in_search = false
+	user_can_delete = false
 
 	add_input_port(
 		DeckType.Types.ANY,