### Godot version

v4.2.dev.custom_build [0c2144da9]

### System information

Debian Linux X11 Xfce

### Issue description

Prerequisite: Be in Single Window Mode in the Editor
The Filesystem Dock is unusable when it is floating, which is not expected.
It appears garbled in the top-left corner:
![image](https://github.com/godotengine/godot/assets/6299227/109f7922-97de-44c3-814a-ca7f970e0602)

The following errors are shown:
```
  editor/window_wrapper.cpp:220 - Condition "!is_window_available()" is true.
  editor/window_wrapper.cpp:202 - Condition "!get_window_enabled()" is true. Returning: Rect2i()
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
  editor/window_wrapper.cpp:207 - Condition "!get_window_enabled()" is true. Returning: -1
```

bisecting lead to #62378
possibly related to #44439

### Steps to reproduce

1. Start a project in the Editor
2. Switch to Single Window Mode in the Editor and reload (if necessary)
3. Make the Filesystem Dock floating

### Minimal reproduction project

Editor issue
if (owner) {
					Ref<SceneState> ss_inst = owner->get_scene_instance_state();
					if (ss_inst.is_valid()) {
						ss_inst->update_instance_resource(p_instance_path, current_packed_scene);
					}
					owner->set_editable_instance(instantiated_node, is_editable);
				}
				
				for (AdditiveNodeEntry additive_node_entry : addition_list) {
					Node *parent_node = instantiated_node->get_node_or_null(additive_node_entry.parent);
					if (!parent_node) {
						parent_node = current_edited_scene;
					}
					parent_node->add_child(additive_node_entry.node);
					parent_node->move_child(additive_node_entry.node, additive_node_entry.index);
					
					if (additive_node_entry.owner == original_node) {
						additive_node_entry.owner = instantiated_node;
					}
					additive_node_entry.node->set_owner(additive_node_entry.owner);
					
					{
						Node2D *node_2d = Object::cast_to<Node2D>(additive_node_entry.node);
						if (node_2d) {
							node_2d->set_transform(additive_node_entry.transform_2d);
						}
						Node3D *node_3d = Object::cast_to<Node3D>(additive_node_entry.node);
						if (node_3d) {
							node_3d->set_transform(additive_node_entry.transform_3d);
						}