Backdrop

SceneManagerBackdrop is a custom Control class to add a child control to show and hide like a modal. If the first child is not your modal node, it won't work because get_child(0) is used to get the node.

You should add backdrop with the screen size. When you click on backdrop, you close the modal. You can change this behavior by disabling close_modal_on_clicked and connecting close_modal_requestedto a method.

Your scene node tree should be similar to the following:

Control (control.gd attached)
|- MainControl (Control type)
|- SceneManagerBackdrop (Anchor Preset: Full Rect)
   |- Modal (Control type)

#control.gd
extends Control

@onready var open_button: Button = $MainControl/OpenButton
@onready var backdrop: SceneManagerBackdrop = $SceneManagerBackdrop

func _ready() -> void:
    open_button.pressed.connect(_on_open_modal)

func _on_open_modal() -> void:
    backdrop.open_modal().finished.connect(_on_modal_opened)

func _on_modal_opened() -> void:
    print("Modal open!")

Check res://addons/scene_manager/test/modals/modals.tscn to see an example.

Animation

Backdrop starts a tween to change opacity and position. Check Animation property group in the inspector to see all the options.