/// Subclass RichTextBox to add the capability to bind scrolling for multiple RichTextBoxs.
   /// This is useful for 'parallel' RTBs that require synchronized scrolling.
   /// Taken from https://gist.github.com/593809
   /// Added WM_HSCROLL 
   /// Added BindScroll() to form a two-way linkage between RichTextBoxes.
   /// Example usage showing how to bind 3 RichTextBoxes together:
   ///    rtb1.BindScroll(rtb2);
   ///    rtb2.BindScroll(rtb3);
   ///    rtb3.BindScroll(rtb1);
   class RichTextBoxSynchronizedScroll : RichTextBox
   {

      private const int WM_VSCROLL = 0x115;
      private const int WM_HSCROLL = 0x114;

      private List<RichTextBoxSynchronizedScroll> peers = new List<RichTextBoxSynchronizedScroll>();

      /// <summary>
      /// Establish a 2-way binding between RTBs for scrolling.
      /// </summary>
      /// <param name="arg">Another RTB</param>
      public void BindScroll( RichTextBoxSynchronizedScroll arg )
      {
         if ( peers.Contains( arg ) || arg==this ) { return; }
         peers.Add( arg );
         arg.BindScroll(this);
      }

      private void DirectWndProc(ref Message m)
      {
         base.WndProc(ref m);
      }

      protected override void WndProc(ref Message m)
      {
         if (m.Msg == WM_VSCROLL || m.Msg == WM_HSCROLL )
         {
            foreach (RichTextBoxSynchronizedScroll peer in this.peers)
            {
               Message peerMessage = Message.Create(peer.Handle, m.Msg, m.WParam, m.LParam);
               peer.DirectWndProc(ref peerMessage);
            }
         }

         base.WndProc(ref m);
      }
   }

/gist.github.com/593809

This is useful for WinForm apps with multiple TextBoxes that need to be scrolled in unison. Similar applications are diff GUIs that show 2 or more files side-by-side, where each window needs to be scrolled in unison.

The approach below is a simpler alternative to http://stackoverflow.com/questions/1827323/c-synchronize-scroll-position-of-two-richtextboxes

Scroll multiple RichTextBoxes (or TextBoxes) in unison (synchronized scrolling).

C#