GitHub - Header Sticky On Top.js

// ==UserScript==
// @name         GitHub - Header Sticky On Top
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       Vic P. @ https://vic.onl/
// @match        https://github.com
// @match        https://github.com/*/*
// @match        https://gist.github.com/*
// @exclude      https://gist.github.com/*/*
// @icon         https://github.githubassets.com/pinned-octocat.svg
// @grant        none
// ==/UserScript==

let get_header_style = height => `
body {
	padding-top: ${height}px;
}

.js-header-wrapper {
	z-index: 120;
	position: fixed !important;
	width: 100%;
	min-width: 1020px;
	top: 0;
	left: 0;
	transform: translate3d(0, 0, 0);
	transition: transform .3s;
}

.js-header-wrapper.hidden {
	transform: translate3d(0, -${height}px, 0);
}`;

(function() {
    'use strict';
    let header = document.querySelector('.js-header-wrapper');
    document.querySelector('head').insertAdjacentHTML('beforeend', `<style>${get_header_style(header.offsetHeight)}</style>`);
    window.addEventListener('scroll', () => header.classList.remove('hidden'));
    header.addEventListener('dblclick', () => window.scroll({ // use native smooth scrolling (Chrome >= 61, Firefox >= 36)
        top: 0,
        behavior: 'smooth'
    }));
})();