source: branches/PersistenceSpeedUp/HeuristicLab.ExtLibs/HeuristicLab.WinFormsUI/2.3.1/WinFormsUI-2.3.1/Docking/DragForm.cs

using System;
using System.Windows.Forms;

namespace WeifenLuo.WinFormsUI.Docking
{
  // Inspired by Chris Sano's article:
  // http://msdn.microsoft.com/smartclient/default.aspx?pull=/library/en-us/dnwinforms/html/colorpicker.asp
  // In Sano's article, the DragForm needs to meet the following criteria:
  // (1) it was not to show up in the task bar;
  //     ShowInTaskBar = false
  // (2) it needed to be the top-most window;
  //     TopMost = true (not necessary here)
  // (3) its icon could not show up in the ALT+TAB window if the user pressed ALT+TAB during a drag-and-drop;
  //     FormBorderStyle = FormBorderStyle.None;
  //     Create with WS_EX_TOOLWINDOW window style.
  //     Compares with the solution in the artile by setting FormBorderStyle as FixedToolWindow,
  //     and then clip the window caption and border, this way is much simplier.
  // (4) it was not to steal focus from the application when displayed.
  //     User Win32 ShowWindow API with SW_SHOWNOACTIVATE
  // In addition, this form should only for display and therefore should act as transparent, otherwise
  // WindowFromPoint will return this form, instead of the control beneath. Need BOTH of the following to
  // achieve this (don't know why, spent hours to try it out :( ):
  //  1. Enabled = false;
  //  2. WM_NCHITTEST returns HTTRANSPARENT
  internal class DragForm : Form
  {
    public DragForm()
    {
      FormBorderStyle = FormBorderStyle.None;
      ShowInTaskbar = false;
      SetStyle(ControlStyles.Selectable, false);
      Enabled = false;
    }

    protected override CreateParams CreateParams
    {
      get
      {
        CreateParams createParams = base.CreateParams;
        createParams.ExStyle |= (int)Win32.WindowExStyles.WS_EX_TOOLWINDOW;
        return createParams;
      }
    }

    protected override void WndProc(ref Message m)
    {
      if (m.Msg == (int)Win32.Msgs.WM_NCHITTEST)
      {
        m.Result = (IntPtr)Win32.HitTest.HTTRANSPARENT;
        return;
      }

      base.WndProc (ref m);
    }

    public virtual void Show(bool bActivate)
    {
      if (bActivate)
        Show();
      else
        NativeMethods.ShowWindow(Handle, (int)Win32.ShowWindowStyles.SW_SHOWNOACTIVATE);
    }
  }
}