I made a userscript that allows the user to toggle the visibility of the sidebar:

    // ==UserScript==
    // @name         Toggle the right sidebar
    // @version      0.3
    // @author       double-beep
    // @include      /^*://(?:[^/]+\.)?(?:(?:stackoverflow|serverfault|superuser|stackexchange|askubuntu|stackapps)\.com|mathoverflow\.net)/(questions/\d+/|review/*)
    // @exclude      /^*://*.*/review/(suggested-edits|reopen)/*
    // @updateURL    https://github.com/double-beep/my-userscripts/raw/master/toggleSidebar.user.js
    // @grant        none
    // @run-at       document-end
    // ==/UserScript==
    
    (function($) {
      $('.-secondary').append('<li class="-item"><a class="-link" id="hide-sidebar" title="Toggle visibility of right sidebar">&#10095;</a></li>');
    
      function defineOnClick() {
        const sidebar = $('#sidebar, .sidebar'), mainbar = $('#mainbar, .mainbar');
        $(document).on('click', '#hide-sidebar', function() {
          if (sidebar.is(':visible')) {
            sidebar.hide(200);
            mainbar.animate({'width': '100%'}, 200);
            $(this).html('&#10094;');
          } else {
            sidebar.show(200);
            mainbar.css('width', 'calc(-324px + 100%)');
            $(this).html('&#10095;');
          }
        });
      }
      if (window.location.pathname.match(/\/questions\/\d+/)) defineOnClick();
    
      var origOpen = XMLHttpRequest.prototype.open;
      XMLHttpRequest.prototype.open = function() {
        this.addEventListener('load', () => {
          if (this.responseURL.match(/\/review\/(next-task|task-reviewed)/)) defineOnClick();
        });
        origOpen.apply(this, arguments);
      };
    })(jQuery);