Option Explicit

Declare Function GetWindowLong Lib "user32" Alias _
      "GetWindowLongA" (ByVal hwnd As Long, ByVal _
      nIndex As Long) As Long

Declare Function SetWindowLong Lib "user32" Alias _
      "SetWindowLongA" (ByVal hwnd As Long, ByVal _
      nIndex As Long, ByVal dwNewLong As Long) As Long

Declare Function SetWindowPos Lib "user32" (ByVal _
      hwnd As Long, ByVal hWndInsertAfter As Long, _
      ByVal X As Long, ByVal Y As Long, ByVal cx As Long, _
      ByVal cy As Long, ByVal uFlags As Long) As Long

Public Const GWL_EXSTYLE = -20
Public Const SWP_NOSIZE = &H1
Public Const SWP_NOMOVE = &H2
Public Const SWP_NOZORDER = &H4
Public Const SWP_NOACTIVATE = &H10
Public Const SWP_FRAMECHANGED = &H20
Public Const WS_EX_CLIENTEDGE = &H200&

Public Sub FormVertiefen(Form As Form, vertiefen As Boolean)
  Dim nWindowLong As Long

  With Form
    nWindowLong = GetWindowLong(.hwnd, GWL_EXSTYLE)

    If vertiefen Then
      nWindowLong = nWindowLong Or WS_EX_CLIENTEDGE
    Else
      nWindowLong = nWindowLong And Not WS_EX_CLIENTEDGE
    End If
    SetWindowLong .hwnd, GWL_EXSTYLE, nWindowLong

    RedrawWindow .hwnd
  End With
End Sub

Public Sub RedrawWindow(ByVal hwnd As Long)
  Const Flags As Long = SWP_NOSIZE Or SWP_NOMOVE Or _
      SWP_NOZORDER Or SWP_NOACTIVATE Or SWP_FRAMECHANGED
  SetWindowPos hwnd, 0, 0, 0, 0, 0, Flags
End Sub

Option Explicit

Private Sub Check1_Click()
  If Check1.Value = 0 Then
    Call FormVertiefen(Form1, False)
  Else
    Call FormVertiefen(Form1, True)
  End If
End Sub