gistfile1.txt

{
    "manifest_version": 2,
    "content_scripts": [ {
        "exclude_globs":    [  ],
        "include_globs":    [  ],
        "js":               [ "github-review.user.js" ],
        "matches":          [   "https://*.github.com/*",
                                "https://github.com/*"
                            ],
        "run_at": "document_end"
    } ],
    "description":  "",
    "name":         "Github Review",
    "version":      "1"
}

github-review.user.js

// ==UserScript==
// @match https://*.github.com/*
// @match https://github.com/*
// ==/UserScript==
//
// Drag/drop into Chrome chrome://extensions/ tab to install.
// Buttons appear in the code review gear popup dialog.

(function(){

var register = function() {
    // console.log("register");
    var helpers = document.querySelector('p#github-review-helpers');
    if (helpers) {
        // don't double register
        return true;
    }
    var whitespaceCheckbox = document.querySelector('input[name=w]');
    if (!whitespaceCheckbox) {
        // don't register if we can't find our checkbox
        return false;
    }

    var markViewed = function(path, viewed) {
        if (!path) { return; }
        var checkboxes = document.querySelectorAll(
            '.file-header[data-path*="' + path + '"] ' +
            'input.js-reviewed-checkbox[type="checkbox"]' + (viewed ? ":not(:checked)" : ":checked")
        );
        checkboxes.forEach(function(i){i.click();});
    };
    var markExpanded = function(path, expanded) {
        if (!path) { return; }
        var toggles = document.querySelectorAll(
            ' .file' + (expanded ? ':not(.open)':'.open') +
            ' .file-header[data-path*="' + path + '"]' +
            ' button.js-details-target[aria-expanded]'
        );
        toggles.forEach(function(i){i.click();});
    };

    var p = document.createElement("p");
    p.id = "github-review-helpers";
    p.style.display = "block";
    p.style.textAlign = "center";

    var appendButton = function(label, f) {
        var b = document.createElement("button");
        b.style.padding="0px 2px";
        b.innerText=label;
        b.className="btn btn-sm";
        p.appendChild(b);
        b.onclick = function(event) {
            event.preventDefault();
            var path = prompt("Path","");
            if (path) { f(path); }
        };
    };

    appendButton("View",    function(path) { markViewed(path,true);    });
    appendButton("Unview",  function(path) { markViewed(path,false);   });
    appendButton("Expand",  function(path) { markExpanded(path,true);  });
    appendButton("Collapse",function(path) { markExpanded(path,false); });

    whitespaceCheckbox.insertAdjacentElement("beforebegin", p);
    return true;
};
// register at start, or keep attempting until we succeed
if (register()) {
    // console.log("success");
} else {
    var interval;
    interval = window.setInterval(function(){
        if (register()) {
            window.clearInterval(interval);
            // console.log("success");
        }
    }, 1000);
}

})();