ToggleMessengerSidebarNice.user.js

// ==UserScript==
// @name Toggle Messenger Sidebar Nice
// @version 0.5
// @description Button to hide messengers list of conversations
// @match https://www.messenger.com/*
// @copyright 2018+, BenCOXXX
// @require http://code.jquery.com/jquery-latest.js
// @resource openImg http://downloadicons.net/sites/default/files/shops-open-icon-65322.png
// @resource closeImg http://downloadicons.net/sites/default/files/shops-close-icon-65321.png
// @grant GM_getResourceURL
// ==/UserScript==

$(document).ready(function() {
    var convThreads = document.getElementsByClassName("_1enh")[0];
	var time = function(self) {
		if($(self).attr("class") == '_1enh') return 0; else return 1000;
	};
	var refreshButton = function() {
		$("#showHideButton").remove();
		setTimeout(createShowHideButton, time(this));
    };
    $(convThreads).click(refreshButton);
	$(document).on('click', '._gs4', refreshButton);
    var createPosition = function(cls) {
        var prt = document.getElementsByClassName(cls)[0];
        var pos = $('<div class="_1cwz"></div>');
        $(prt).prepend(pos);
        return pos;
    };
    var createImage = function(image, src, alt) {
        var img = image.replace('${src}', src);
        img = img.replace('${alt}', alt);
        return img;
    };
    var createShowHideButton = function() {
       var position = document.getElementsByClassName("_1cwz")[0];
       if(!position) position = createPosition("_673w");
       var div = $('<div/>', {id: 'showHideButton'});
       var openUrl = GM_getResourceURL('openImg');
       var closeUrl = GM_getResourceURL('closeImg');
       var img = '<img src="${src}" alt="${alt}" height="30" width="30" style="filter: invert(1);">';
       $(div).prepend(createImage(img, openUrl, 'Hide Sidebar'));
       $(position).prepend(div);
       $("#showHideButton").css({
           "background-color": $("div._673w > ul > li:nth-child(3) > a > div > svg").css("stroke"),
           "color": "#FFFFFF",
           "left":60,
           "z-index":1000,
           "border-radius":"1.3em",
           "font-size": "12px",
           //"box-shadow": "0px 0px 16px 0px black",
           "border-style": "none",
           "padding": "6px"
       });
       $("#showHideButton").click(function(){
           if(convThreads.style.display == 'none'){
               convThreads.style.display = 'block';
               $("#showHideButton").html(createImage(img, openUrl, 'Hide Sidebar'));
           } else {
               convThreads.style.display = 'none';
               $("#showHideButton").html(createImage(img, closeUrl, 'Show Sidebar'));
           }
       });
    }
    createShowHideButton();
});