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hjkl.user.js

(function() {
    function _onkeypress(key) {
        switch (key.key) {
        case "h":
        case "H":
            window.scrollBy(-100, 0);
            return false;
        case "j":
        case "J":
            window.scrollBy(0, 100);
            return false;
        case "k":
        case "K":
            window.scrollBy(0, -100);
            return false;
        case "l":
        case "L":
            window.scrollBy(100, 0);
            return false;
        }
        return true;   
    }
    window.addEventListener("keypress", _onkeypress);
})();