Can I find help adding a rule to my Karabiner json file

Question

I'm 85 years old. Back in the sixties I actually programmed mainframe computers in assembler language, but those days are long gone. Now I have a 2018 Mac Mini, a DasKeyboard MacSFT 3G7 keyboard, a Kensington Expert Trackball, and a new Wacom One screen tablet.

I'm currently running Mac OS 10.15.7 (but I realize I need to update to the current OS) and Karabiner Elements 13.7.0. I want to put Affinity Photo on the Wacom screen and at the same time run YouTube tutorials on the monitor screen. Touching the pen to the tablet moves the cursor to the tablet, but I want to be able to press the #3 (upper left) button on my trackball and have it move the cursor to the center of the main monitor when I need to pause the tutorial, etc..

So far I have come up with code for a rule which maybe would do this (using the "to" code from an example at set_mouse_cursor_position):

[
    {
        "type": "basic",
        "from": {
            "pointing_button": 3
        },     [<– This is the only thing I added]
    
        "to": [
            {
                "software_function": {
                    "set_mouse_cursor_position": {
                        "x": "50%",
                        "y": "50%",
                        "screen": 0
                    }
                }
            }
        ]
    }
]

But I don't know how to test this and I don't know how to add it to the Karabiner [JSON?] file and then I don't know how to get that file where it will actually do its magic.

Is there anyone here who could test out this rule (and correct any mistakes), add it to the Karabiner Elements complex modifications rules list, and then tell me here where it is and step-by-step how to add it to the Karabiner file/app on my computer.

Answer 1

Karabiner's config file is ~/.config/karabiner/karabiner.json. If this file doesn't exist try to add any Complex Modification in Karabiner app via UI just to trigger the creation of the file.

You should add your custom rule to profiles.complex_modifications.rules JSON property.

Unfortunately I can't test your rule right now but if it won't work you should start with some basic mapping (like from one key to another) and make it more complex step-by-step.