self-reply.user.js

// ==UserScript==
// @name         Self Reply
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Adds button to reply to self
// @author       Redwolf Programs (Ryan Tosh)
// @match        https://chat.stackexchange.com/rooms/*
// @match        https://chat.stackoverflow.com/rooms/*
// @match        https://chat.meta.stackexchange.com/rooms/*
// @grant        none
// ==/UserScript==

(() => {
    var style = document.createElement("style");

    document.head.appendChild(style);

    style.sheet.insertRule("html #chat-body .monologue.mine:hover .message .meta { display: inline-block; }");
    style.sheet.insertRule("html #chat-body .monologue .meta { background-color: inherit; }");
    style.sheet.insertRule(".monologue .message { background-color: inherit; }");
    style.sheet.insertRule("html #chat-body .monologue.mine .newreply { filter: grayscale(100%) contrast(0%) brightness(0%); opacity: 20%; }");

    var observer = new MutationObserver((records) => {
        var span;

        var data = records.filter(r => r.type == "childList" && r.target.parentNode).reduce((d, r) => [...d, ...[...r.target.querySelectorAll("span.meta")].filter(t => !d.find(i => i == t))], []);

        for (var i = 0; i < data.length; i++) {
            if ([...data[i].childNodes].some(n => n.className == "newreply"))
                continue;

            span = document.createElement("span");

            span.className = "newreply";
            span.title = "link my next chat message as a reply to this";

            span.onclick = ((span) => () => {
                var input = document.getElementById("input");

                input.focus();

                input.value = ":" + span.parentNode.parentNode.id.split("-")[1] + " " + input.value.replace(/^:([0-9]+)\s+/, "");
            })(span);

            while (data[i].firstChild)
                data[i].removeChild(data[i].firstChild);

            data[i].appendChild(span);
        }
    });

    observer.observe(document.getElementById("chat"), {attributes: false, childList: true, subtree: true});
})();