Hello. Thanks.
The game does not have a "Borderless Windowed" setting.
I'm not sure what artifacts you're referring to, but I think we're talking about that weird artifacts that happens in the game if you run it in OpenGL mode. In any other case I can't help you. Check your card.
You can find the config here:
C:\Users\USERNAME\AppData\Roaming\Godot\app_userdata\Lilrien's Secret Potions\saves. The value "fs_mode" controls the window mode.