Public Class Form1
    Declare Function GetNextWindow Lib "user32" Alias "GetWindow" (ByVal hwnd _
      As IntPtr, ByVal wFlag As Int32) As IntPtr
    Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd _
    As IntPtr, ByVal wMsg As Integer, _
                                 ByVal wParam As Integer, ByVal lParam As _
                                 String) As Integer
    Declare Function GetForegroundWindow Lib "user32" () As IntPtr
    Const SB_DOWN As Int32 = 1           ' 1 Zeile runter
    Const SB_UP As Int32 = 0             ' 1 Zeile hoch
    Const SB_BOTTOM As Int32 = 7         ' ganz nach unten
    Const WM_VSCROLL As Int32 = &H115
    Const GW_CHILD As Int32 = 5
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        TextBox1.ScrollBars = ScrollBars.Vertical
        For i As Int32 = 1 To 50
            TextBox1.AppendText(i.ToString & vbNewLine)
        Next
    End Sub
 
    Private Sub Button1_MouseDown(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.MouseEventArgs) Handles Button1.MouseDown
        Dim hwnd As IntPtr = GetForegroundWindow                                _
             ' das aktive Hauptfenster
        If e.Button = Windows.Forms.MouseButtons.Left Then
            SendMessage(GetNextWindow(hwnd, GW_CHILD), WM_VSCROLL, SB_DOWN, _
              Nothing) ' Textbox = Childfenster
        Else
            SendMessage(GetNextWindow(hwnd, GW_CHILD), WM_VSCROLL, SB_UP, _
              Nothing)
        End If
    End Sub
End Class