Scroll comments in Github (tampermonkey)

// ==UserScript==
// @name        Prev/Next Github comment
// @namespace   justinjc
// @description Userscript that adds prev/next comment hotkeys for Github.
// @include     https://github.com*
// @version     0.1
// @author      Justin Chan
// @copyright   2019 Justin Chan
// ==/UserScript==

(function() {
    'use strict';

    var modifier = "Control";
    var prevKeyCode = 74; // k
    var nextKeyCode = 75; // j

    // Modifier to align the top of a comment at what height on the screen
    // 0 = top of screen; 1 = bottom of screen
    var screenOffset = 0.3;

    function nextCommentIdx(comments, currY) {
        var center = currY + scrollOffset()

        for (var i = 0; i < comments.length; i++) {
            var commentY = posY(comments[i]);
            if (commentY == currY || commentY <= center) {
                continue;
            }
            return i;
        }
        return -1;
    }

    function prevCommentIdx(comments, currY) {
        var center = currY + scrollOffset()

        for (var i = comments.length - 1; i >= 0; i--) {
            var commentY = posY(comments[i]);
            if (commentY == currY || commentY >= center) {
                continue;
            }
            return i;
        }
        return -1;
    }

    function posY(ele) {
        var box = ele.getBoundingClientRect();

        var body = document.body;
        var docEl = document.documentElement;

        var scrollTop = window.pageYOffset || docEl.scrollTop || body.scrollTop;
        var clientTop = docEl.clientTop || body.clientTop || 0;

        var top  = box.top +  scrollTop - clientTop;

        return Math.round(top);
    }

    function scrolled(origY) {
        return origY != window.pageYOffset;
    }

    function scrollOffset() {
        return Math.floor(window.innerHeight * screenOffset);
    }

    function gotoComment(e) {
        var comments = document.getElementsByClassName("js-resolvable-thread-contents");
        var origY = window.pageYOffset;

        if (e.getModifierState(modifier) && e.keyCode === prevKeyCode) {
            nextComment(comments, origY)
        }

        if (e.getModifierState(modifier) && e.keyCode === nextKeyCode) {
            prevComment(comments, origY)
        }
    }

    function nextComment(comments, origY) {
        var idx = nextCommentIdx(comments, origY);
        if (idx == -1) {
            return;
        }
        for (var i = idx; i < comments.length; i++) {
            comments[i].scrollIntoView();
            if (scrolled(origY)) {
                window.scrollBy(0, -scrollOffset())
                return;
            }
        }
    }

    function prevComment(comments, origY) {
        var idx = prevCommentIdx(comments, origY);
        if (idx == -1) {
            return;
        }
        for (var i = idx; i >= 0; i--) {
            comments[i].scrollIntoView();
            if (scrolled(origY)) {
                window.scrollBy(0, -scrollOffset())
                return;
            }
        }
    }

    document.addEventListener('keydown', gotoComment);
})();