comment_expander.user.js

// ==UserScript==
// @name       Github Expand All Comments
// @namespace  http://www.edx.org/developers
// @version    0.1
// @description  Help manage github pull request comments
// @match      *://github.com/*/pull/*
// @copyright  2014+, Gabe Mulley
// @require    http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js
// ==/UserScript==

menuobj = document.createElement('div');
menuobj.style.position = 'fixed';
menuobj.style.top = '10px';
menuobj.style.left = '10px';
menuobj.style.backgroundColor = '#ffffff00';
menuobj.innerHTML = '<input id="toggle-all-comments" type="button" value="Show All Comments" />';
body = document.getElementsByTagName('body')[0];
body.appendChild(menuobj);

$('#toggle-all-comments').click(function(e) {
    $('.discussion-item-toggle-closed').each(function (i, el) {
        var evt = document.createEvent("MouseEvents");
        evt.initEvent("click", true, true);
        el.dispatchEvent(evt);
    });
    button = $( this )
    if (button.prop('value') == 'Show All Comments') {
        button.prop('value', 'Hide All Comments');
    } else {
        button.prop('value', 'Show All Comments');
    }
});