Tabbed Rendering Selector

SelectRendering.xml

<?xml version="1.0" encoding="utf-8" ?>
<control xmlns:def="Definition" xmlns="http://schemas.sitecore.net/Visual-Studio-Intellisense">
  <Sitecore.Shell.Applications.Dialogs.SelectRendering>
    <FormDialog ID="Dialog" Icon="Core/32x32/open_document.png" Header="Open Item" 
      Text="Select the item that you wish to open. Then click the Open button." OKButton="Open">
      <Stylesheet Src="SelectItemWithThumbnails.css" DeviceDependant="true" />
      <CodeBeside Type="MyProject.CMS.Custom.Dialogs.SelectRenderingTabbed.SelectRenderingForm, ARM.CMS.Custom"/>
      
      <DataContext ID="DataContext" Root="/"/>

      <GridPanel Width="100%" Height="100%">

        <GridPanel ID="RenderingSelector" Width="100%" Height="0%" Columns="3" GridPanel.Height="0%" Style="display:none">
          
          <Scrollbox Width="100%" ID="TreeviewContainer" Height="100%" Class="scScrollbox scFixSize scFixSize4" style="border-right: 2px solid #474747;" Background="white" Padding="0px" GridPanel.Width="200px" GridPanel.Height="100%">
            <TreeviewEx ID="Treeview" DataContext="DataContext" ShowRoot="true" Click="Treeview_Click" />
          </Scrollbox>

          <VSplitter ID="TreeSplitter" GridPanel.Class="scThinSplitter" Target="left" />
          
          <Scrollbox ID="Renderings" Width="100%" Height="100%" Class="scScrollbox scFixSize scFixSize4" Background="white" Padding="0px" GridPanel.Height="100%">            
          </Scrollbox>
          
        </GridPanel>

        <GridPanel ID="TabbedSelector" Width="100%" Height="100%" Columns="1" GridPanel.Height="100%" Style="table-layout:fixed">

          <Tabstrip ID="Tabs" Width="100%" Height="100%">
          </Tabstrip>

        </GridPanel>
                       
        <Border ID="PlaceHolderNameBorder" Visible="false" style="padding:12px 0px 0px 0px">
          <GridPanel Columns="3" Width="100%">
            <Literal Text="Add to Placeholder: " GridPanel.NoWrap="true" />
            <Space Width="4" />
            <Edit ID="PlaceholderName" Name="PlaceholderName" GridPanel.Width="100%" class="scQuirksBoxModel" Width="100%" />
          </GridPanel>
        </Border>
       
      </GridPanel>

      <Border ID="OpenPropertiesBorder" def:placeholder="Buttons" Visible="false" style="padding:0px 0px 8px 0px">
        <Checkbox ID="OpenProperties" Header="Open the Properties dialog box immediately." />
      </Border>  
    </FormDialog>
  </Sitecore.Shell.Applications.Dialogs.SelectRendering>
</control>

SelectRenderingForm.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Web.UI;
using System.Web.UI.WebControls;
using Sitecore;
using Sitecore.Collections;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.Form.UI.Controls;
using Sitecore.Shell.Applications.Dialogs.ItemLister;
using Sitecore.Web.UI.HtmlControls;
using Sitecore.Web.UI.WebControls;

namespace MyProject.CMS.Custom.Dialogs.SelectRenderingTabbed
{
    [UsedImplicitly]
    public class SelectRenderingForm : Sitecore.Shell.Applications.Dialogs.SelectRendering.SelectRenderingForm
    {
        protected Tabstrip Tabs;
        protected GridPanel RenderingSelector;
        protected GridPanel TabbedSelector;

        protected override void OnLoad(EventArgs e)
        {
            Assert.ArgumentNotNull((object) e, "e");
            IsOpenPropertiesChecked = Registry.GetBool("/Current_User/SelectRendering/IsOpenPropertiesChecked");
            var renderingOptions = SelectItemOptions.Parse<SelectRenderingOptions>();

            base.OnLoad(e);
            if (Sitecore.Context.ClientPage.IsEvent)
                return;

            if (!renderingOptions.ShowTree)
            {
                base.Renderings.InnerHtml = String.Empty; //reset the rendering listing
                RenderingSelector.Visible = false;

                var grouppedSublayouts = renderingOptions.Items.GroupBy(i => i.Parent.Name);

                //Add new tab for each folder
                foreach (IGrouping<string, Item> grouppedSublayout in grouppedSublayouts)
                {
                    var newTab = new Tab();
                    newTab.Header = grouppedSublayout.Key;
                    var newScrollbox = new Scrollbox();
                    newScrollbox.Class = "scScrollbox scFixSize scFixSize4";
                    newScrollbox.Background = "white";
                    newScrollbox.Padding = "0px";
                    newScrollbox.Width = new Unit(100, UnitType.Percentage);
                    newScrollbox.Height = new Unit(100, UnitType.Percentage);

                    string text = RenderPreviews(grouppedSublayout.ToList());

                    newScrollbox.InnerHtml = text;
                    newTab.Controls.Add(newScrollbox);
                    Tabs.Controls.Add(newTab);
                }
            }
        }

        private string RenderPreviews(IEnumerable<Item> items)
        {
            Assert.ArgumentNotNull((object) items, "items");
            HtmlTextWriter output = new HtmlTextWriter((TextWriter) new StringWriter());
            foreach (Item obj in items)
                this.RenderItemPreview(obj, output);
            return output.InnerWriter.ToString();
        }
    }
}