If I’m Linus, i would have removed bcachefs from mainline already. From his reply he never once admit his attitude problem and keep thinking he is right.

This type of people can’t work with others, not today, not in future.

You can write a Kwin script and connect to the windowActivated function. Unfortunately you can’t run arbitrary command from Kwin script, but as suggested in the comment you can use DBus to workaround that.

workspace.windowActivated.connect(function(client) {
    if (client != null && client.caption == "name") {
        callDBus(...); // call your DBus function which call ddcutil
    }
})

Just an example, you will have to do some trial and error yourself!

what’s the exact prompt?