If you use only one channel in slack or your window is narrow, you may want to hide Slacks sidebar.

slackhidesidebar.user.js

// ==UserScript==
// @name          Slack: Hide sidebar
// @namespace     http://userstyles.org
// @description	  Slack doesn't support narrow windows that good. This hides the sidebar.
// @author        Roman Karwacik
// @run-at        document-start
// @match         https://*.slack.com/*
// @version       1
// ==/UserScript==
(function() {var css = [
    ".client_channels_list_container {display: none;}"
].join("\n");
if (typeof GM_addStyle != "undefined") {
	GM_addStyle(css);
} else if (typeof PRO_addStyle != "undefined") {
	PRO_addStyle(css);
} else if (typeof addStyle != "undefined") {
	addStyle(css);
} else {
	var node = document.createElement("style");
	node.type = "text/css";
	node.appendChild(document.createTextNode(css));
	var heads = document.getElementsByTagName("head");
	if (heads.length > 0) {
		heads[0].appendChild(node);
	} else {
		// no head yet, stick it whereever
		document.documentElement.appendChild(node);
	}
}
})();