source: stable/HeuristicLab.ExtLibs/HeuristicLab.WinFormsUI/2.7.0/WinFormsUI-2.7.0/Docking/SplitterBase.cs

using System;
using System.Collections;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;

namespace WeifenLuo.WinFormsUI.Docking
{
  internal class SplitterBase : Control
  {
    public SplitterBase()
    {
      SetStyle(ControlStyles.Selectable, false);
    }

    public override DockStyle Dock
    {
      get { return base.Dock; }
      set
      {
        SuspendLayout();
        base.Dock = value;

        if (Dock == DockStyle.Left || Dock == DockStyle.Right)
          Width = SplitterSize;
        else if (Dock == DockStyle.Top || Dock == DockStyle.Bottom)
          Height = SplitterSize;
        else
          Bounds = Rectangle.Empty;

        if (Dock == DockStyle.Left || Dock == DockStyle.Right)
          Cursor = Cursors.VSplit;
        else if (Dock == DockStyle.Top || Dock == DockStyle.Bottom)
          Cursor = Cursors.HSplit;
        else
          Cursor = Cursors.Default;
          
        ResumeLayout();
      }
    }

    protected virtual int SplitterSize
    {
      get { return 0; }
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
      base.OnMouseDown(e);

      if (e.Button != MouseButtons.Left)
        return;

      StartDrag();
    }

    protected virtual void StartDrag()
    {
    }

    protected override void WndProc(ref Message m)
    {
            // eat the WM_MOUSEACTIVATE message
      if (m.Msg == (int)Win32.Msgs.WM_MOUSEACTIVATE)
        return;

      base.WndProc(ref m);
    }
  }
}