I made a userscript that allows the user to toggle the visibility of the sidebar (the userscript's source code is also on [GitHub](https://github.com/double-beep/my-userscripts/blob/master/toggleSidebar.user.js)):

    // ==UserScript==
    // @name         Toggle the right sidebar
    // @version      0.5.0
    // @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
    // @downloadURL  https://github.com/double-beep/my-userscripts/raw/master/toggleSidebar.user.js
    // @grant        none
    // ==/UserScript==
    
    (function() {
      const buttonWrapper = document.createElement('li');
      buttonWrapper.classList.add('-item');
      const toggleButton = document.createElement('a');
      toggleButton.classList.add('-link');
      toggleButton.id = 'hide-sidebar';
      toggleButton.title = 'Toggle visibility of right sidebar';
      toggleButton.innerHTML = '❯';
      buttonWrapper.appendChild(toggleButton);
      document.querySelector('.-secondary').appendChild(buttonWrapper);
    
      const sidebar = $(document.querySelector('#sidebar, .sidebar')), mainbar = document.querySelector('#mainbar, .mainbar');
      document.querySelector('#hide-sidebar').addEventListener('click', event => {
        const toggleButton = event.target;
        if (sidebar.is(':visible')) {
          sidebar.hide(200);
          $(mainbar).animate({ width: '100%' }, 200);
          toggleButton.innerHTML = '❮';
        } else {
          sidebar.show(200);
          mainbar.style.width = 'calc(-324px + 100%)';
          toggleButton.innerHTML = '❯';
        }
      });
    })();

The button is in the menu bar:

[![][1]][1]


  [1]: https://i.sstatic.net/ugb2t.png