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

    // ==UserScript==
    // @name         Toggle the right sidebar
    // @version      0.2
    // @author       double-beep
    // @include      /^https://(?:[^/]+\.)?(?:(?:stackoverflow|serverfault|superuser|stackexchange|askubuntu|stackapps)\.com|mathoverflow\.net)/questions/\d+/
    // @updateURL    https://github.com/double-beep/my-userscripts/raw/master/toggleSidebar.user.js
    // @grant        none
    // ==/UserScript==
    
    (function($) {
      const sidebar = $('#sidebar'), mainbar = $('#mainbar');
      $('.-secondary').append('<li class="-item"><a class="-link" id="hide-sidebar" title="Toggle visibility of right sidebar">&#10095;</a></li>');
      $('#hide-sidebar').click(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;');
        }
      });
    })(jQuery);