Making the Messenger sidebar narrow. Adapted the original script to handle dynamic CSS names, and better work with vertical screens. This will compact the sidebar when you keep Messenger window on a vertical HD screen (1080px) or snap its window to a side of a HD (960px) or ultrawide (1280px width available) screen, but will expand it on e.g. HD…

compact-messenger.user.js

// ==UserScript==
// @name         Compact Messenger Sidebar
// @version      2
// @match        https://www.messenger.com/*
// @grant        GM_xmlhttpRequest
// @grant        GM_addStyle
// ==/UserScript==

const targetCSS = document.getElementsByTagName("head")[0].querySelectorAll("link[href*='.css']")[2];
GM_xmlhttpRequest({
    method: "GET",
    url:    targetCSS.href,
    onload: response => {
        targetCSS.parentNode.removeChild(targetCSS);
        GM_addStyle(response.responseText.replace(/max-width: 900px/g, "max-width: 1280px"));
    }
});