I saw this extension and thought it would be possible to create a visual effect
that creates objects outside the screen
You can do some effects - e.g. the window hole in BLAWK is made using this extension (the game takes a screenshot when you start that level and keeps drawing from it).
But if you wanted to do something like inverting colors on a part of the screen in real-time, then no, you can’t quite do that, be it with this extension or in general - Windows only supports capturing part of the screen (which is everything you see, except for the mouse cursor) or a part of a window (which is what’s inside the window, except for the window title/borders)