Skeleton to add a custom button to the Godot editor above the view.

CustomEditorButton.gd

# CustomEditorButton.gd
# Created 2023.08.12 by Brom Bresenham

@tool
extends EditorPlugin

var selected_object:Control
var button:Button

func _handles( object:Object )->bool:
	return object is Control # or whatever type you want to handle

func _edit( object:Object ):
	selected_object = object

func _make_visible( show:bool ):
	if show:
		button = Button.new()
		button.flat = true
		button.text = "Custom Button"
		button.pressed.connect( on_button_pressed )
		add_control_to_container( EditorPlugin.CONTAINER_CANVAS_EDITOR_MENU, button )

	elif button:
		remove_control_from_container( EditorPlugin.CONTAINER_CANVAS_EDITOR_MENU, button )
		button.queue_free()
		button = null

func on_button_pressed():
	print("button pressed")