SlidePanelInjector.cs

using Sitecore.Data;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Shell.Applications.ContentEditor.Pipelines.RenderContentEditor;

namespace SCXM.Panels.Pipelines.RenderContentEditor
{
    public class SlidePanelInjector
    {
        public void Process(RenderContentEditorArgs args)
        {
            Assert.ArgumentNotNull(args, nameof(args));
            
            var style = @"
                <style>
                  #scxmPanel {
                    position: fixed;
                    top: 60px; 
                    right: -300px; 
                    width: 300px;
                    height: calc(100% - 60px);
                    background: #fff;
                    box-shadow: -2px 0 5px rgba(0,0,0,0.2);
                    transition: right 0.3s ease; 
                    z-index: 9998;
                  }

                  #scxmPanel.open {
                    right: 0; 
                  }

                  #openPanel {
                    position: fixed;
                    bottom: 4px;
                    right: 12px;
                    z-index: 9999;
                    background: #fff;
                    border-radius: 4px;
                    border: 1px solid #ccc;
                    padding: 3px 6px;
                    cursor: pointer;
                    box-shadow: 0 2px 5px rgba(0,0,0,0.2);
                    font-size: 10px;
                  }
                </style>
            ";

            var script = @"
              <script>
                (function(){
                  var btn = document.getElementById('openPanel');

                  var panel = document.getElementById('scxmPanel');

                  btn.addEventListener('click', function(){
                    panel.classList.toggle('open');
                  });
                })();
              </script>
            ";

            var injectDoc = $@"				
              {style}

              <div id='scxmPanel'>
                <h3 style='margin:10px;'>Persistent Panel</h3>
                <p> Your info goes here </p>
              </div>

              <div id='openPanel'>☰</div>

              {script}
            ";

            args.EditorFormatter.AddLiteralControl(args.Parent, injectDoc);
        }
    }
}