Python: canvas-flip action is merged with the next user action in undo history

Question

I wrote a simple add-on to enable canvas flipping in Blender (for Grease Pencil) with a hotkey, as well as a menu button. All it does is multiply the camera's local X scale by -1:

current_scene = bpy.context.scene;
active_camera = current_scene.camera;
active_camera.scale.x *= -1;

This basically works. However, if I:

1. flip the canvas
2. draw a Grease Pencil stroke
3. undo with Ctrl+Z

the last stroke disappears as expected, but the canvas also flips back. Somehow both actions are interpreted as one single operation.

(This code is based on a brief description of a similar functionality in the GP Toolbox add-on, which has the same problem.)

Why are the actions combined? Did I miss a step in the code?

Full add-on code below.

bl_info = {
    "name": "Flip Canvas",
    "author": "Latin1",
    "version": (1, 0),
    "blender": (4, 1, 1),
    "location": "Scene",
    "description": "Inverts the camera's x axis",
    "warning": "",
    "doc_url": "",
    "category": "Scene",
}

import bpy;

def flip_canvas_btn(context):
    
    current_scene = bpy.context.scene;
    active_camera = current_scene.camera;
    active_camera.scale.x *= -1;

class Flip_Canvas(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "latin1.flip_canvas";
    bl_label = "Flip canvas";

    def execute(self, context):
        flip_canvas_btn(context);
        return {'FINISHED'};

def menu_func(self, context):
    self.layout.operator(SimpleOperator.bl_idname, text=SimpleOperator.bl_label);


# ----------------------------------------------------------------------------------------
class LayoutDemoPanel(bpy.types.Panel):
    """Creates a Panel in the scene context of the properties editor"""
    bl_label = "Flip the canvas"
    bl_idname = "SCENE_PT_layout"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "scene"

    def draw(self, context):
        layout = self.layout;

        scene = context.scene;

        # Interface button
        row = layout.row();
        row.scale_y = 1.0;
        row.operator("latin1.flip_canvas");


def register():
    bpy.utils.register_class(Flip_Canvas);
    bpy.types.VIEW3D_MT_object.append(menu_func);
    bpy.utils.register_class(LayoutDemoPanel);


def unregister():
    bpy.utils.unregister_class(Flip_Canvas);
    bpy.types.VIEW3D_MT_object.remove(menu_func);
    bpy.utils.unregister_class(LayoutDemoPanel);


if __name__ == "__main__":
    register();

Answer 1

That is because you did not register your operator to be included into the UNDO history. You can do that by adding bl_options = {'REGISTER', 'UNDO'}

class Flip_Canvas(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "latin1.flip_canvas";
    bl_label = "Flip canvas";
    bl_options = {'REGISTER', 'UNDO'}