a.cs

public const int  GWL_STYLE   = -16;
public const int  WS_HSCROLL  = 0x00100000;
public const int  WS_VSCROLL  = 0x00200000;


[ DllImport( "user32.dll", EntryPoint = "GetWindowLong", CharSet = CharSet.Auto ) ]
public static extern IntPtr GetWindowLong32( IntPtr hWnd, int nIndex );

[ DllImport( "user32.dll", EntryPoint = "GetWindowLongPtr", CharSet = CharSet.Auto ) ]
public static extern IntPtr GetWindowLongPtr64( IntPtr hWnd, int nIndex );

[ DllImport( "user32.dll", EntryPoint = "SetWindowLong", CharSet = CharSet.Auto ) ]
public static extern IntPtr SetWindowLongPtr32( IntPtr hWnd, int nIndex, int dwNewLong );

[ DllImport( "user32.dll", EntryPoint = "SetWindowLongPtr", CharSet = CharSet.Auto ) ]
public static extern IntPtr SetWindowLongPtr64( IntPtr hWnd, int nIndex, int dwNewLong );


public static int GetWindowLong( IntPtr hWnd, int nIndex )
{
  if( IntPtr.Size == 4 )
  {
    return (int)GetWindowLong32( hWnd, nIndex );
  }
  else
  {
    return (int)(long)GetWindowLongPtr64( hWnd, nIndex );
  }
}


public static int SetWindowLong( IntPtr hWnd, int nIndex, int dwNewLong )
{
  if( IntPtr.Size == 4 )
  {
    return (int)SetWindowLongPtr32( hWnd, nIndex, dwNewLong );
  }
  else
  {
    return (int)(long)SetWindowLongPtr64( hWnd, nIndex, dwNewLong );
  }
}


protected override void WndProc( ref Message m )
{
  switch( m.Msg )
  {
    case WM_NCCALCSIZE:
    {
      var style = (int)GetWindowLong( this.Handle, GWL_STYLE );

      if( ( style & WS_HSCROLL ) == WS_HSCROLL )
      {
        SetWindowLong( Handle, GWL_STYLE, style & ~WindowsNative.WS_HSCROLL );
      }

      break;
    }
  }

  base.WndProc( ref m );
}