Shows YouTube header when moving mouse to the top of the browser

mousehideytheader.user.js

// ==UserScript==
// @version     7
// @name        Mouseover show Youtube Header
// @namespace   https://github.com/YePpHa/YouTubeCenter/issues/1393
// @description Shows YouTube header when moving the mouse to the top of the browser
// @match       *://www.youtube.com/*
// @run-at      document-start
// @grant       none
// @noframes
// ==/UserScript==
(function () {
    'use strict';
    var sheet =
        '#masthead-positioner,#masthead-positioner-height-offset{margin-top:0;transition:margin-top 1s}\n' +
        '#body #body-container .hide{margin-top:-50px}',
        core = function () {
            var header,
                offset;
            function mouseTrack(a) {
                header = document.getElementById('masthead-positioner');
                offset = document.getElementById('masthead-positioner-height-offset');
                if (location.href.indexOf('/watch') !== -1) {
                    if (a.clientY <= 90 && header && header.classList.contains('hide')) {
                        header.classList.remove('hide');
                        if ((window.chrome && document.body.scrollTop <= 50) || document.documentElement.scrollTop <= 50) {
                            offset.classList.remove('hide');
                        }
                    } else if (a.clientY > 90 && header && !header.classList.contains('hide') && !header.contains(document.activeElement)) {
                        header.classList.add('hide');
                        offset.classList.add('hide');
                    }
                }
            }
            function reset(){
                if (location.href.indexOf('/watch') === -1 && header && header.classList.contains('hide')) {
                    header.classList.remove('hide');
                    offset.classList.remove('hide');
                }
            }
            function scroll(a) {
                if (window.chrome && document.body.scrollTop === 50) {
                    document.body.scrollTop = 0;
                } else if (a.pageY === 50) {
                    document.documentElement.scrollTop = 0;
                }
            }
            window.addEventListener('mousemove', mouseTrack, false);
            window.addEventListener('spfdone', reset, false);
            window.addEventListener('scroll', scroll, true);
        };
    function inject(a, b) {
        var injection = document.createElement(b);
        document.head.appendChild(injection);
        injection.textContent = a;
    }
    inject(sheet, 'style');
    inject('(' + core + ')()', 'script');
}());