How to force an exported variable to be shared in all instances of the scene?

xyz I'm trying to export some variable in a child node, and to do so, I've declared an exported variable inside the root node. Using setter and getter methods I'm updating the child node. But the update does not reflect inside the editor.

I have used @tool to make it update in the editor too. But the variable in the root node is local to instances, and the child node (CollisionShape3D) is a shared resource. Updating the value in an instance inspector does change the child for all instances, but the numbers inside the inspectors remain different.

Using Engine.is_editor_hint inside _process function, I've managed to update the numbers as well, however if I change the number in an instance in another scene and save the project and exit, the values set in the main scene itself will overwrite the values set in the instances. Because I haven't navigated to the main scene before exiting and the _process (and _ready) has not been called.

I'm new to Godot... so sorry if there is any stupidity in my workflow xD

But here are the codes:

@tool
extends Area3D

@export var size = Vector3.ONE:
	get:
		return size
	set(value):
		size = value
		if not is_inside_tree():
			await ready
		get_node("CollisionShape3D").shape.size = size

func _ready():
	if Engine.is_editor_hint():
		if size != get_node("CollisionShape3D").shape.size:
			size = get_node("CollisionShape3D").shape.size
			print("ready")

func _process(_delta):
	if Engine.is_editor_hint():
		if size != get_node("CollisionShape3D").shape.size:
			size = get_node("CollisionShape3D").shape.size
			print("process")

xyz My bad. CollisionShape3D gets a BoxShape3D which I guess is a resource. However it doesn't matter much, because it's shared between the instances and that's the point.

My final goal is to make the size of the BoxShape3D (child of Area3D) editable inside the inspector, without the need to make the children editable in the editor. I want the change to be reflected in the viewport immediately, showing me the bounding box of the BoxShape3D. And I want the exported value to update inside all the instances in the scene immediately.

The problem for the moment is that everything works fine, expect the exported value in the Area3D node. I think the explanation of my previous post now makes sense.

In a nutshell: how am I supposed to make a property of the child node be exposed at the top-level parent.

Thank you for trying to help me!