Синхронная прокрутка в двух RichTextBox - Visual Basic .NET

Public Class Form1
'   Заменяем стандартные названия на эти
    '   A Checkbox called "chk".
    '- A RichTextBox called "RTB1".
    '- A RichTextBox called "RTB2".
 
    ' Constants and SendMessage function
    Const WM_USER As Integer = &H400
    Const EM_GETSCROLLPOS As Integer = WM_USER + 221
    Const EM_SETSCROLLPOS As Integer = WM_USER + 222
    Declare Function SendMessage Lib "user32.dll" Alias "SendMessageW" (ByVal hWnd As IntPtr, ByVal msg As Integer, ByVal wParam As Integer, ByRef lParam As Point) As Integer
 
    ' Scrollpositions of rtb1 and rtb2
    Dim ptScrollPos_RTB1, ptScrollPos_RTB2 As Point
 
    ' Wether the scroll was made by the user (false) or by the synched scrolling function (true)
    Dim blnScrolled_RTB1, blnScrolled_RTB2 As Boolean
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)
 
        ' Load the RTB's with textfile
        Dim strFilePath As String = ""
        Using ofd As New OpenFileDialog With {.Filter = "Text files (*.txt)|*.txt|All files|*.*", .Title = "Open text file"}
            If ofd.ShowDialog = Windows.Forms.DialogResult.OK Then
                strFilePath = ofd.FileName
 
                RTB1.LoadFile(strFilePath, RichTextBoxStreamType.PlainText)
                RTB2.LoadFile(strFilePath, RichTextBoxStreamType.PlainText)
            Else
                MessageBox.Show("You are encouraged to load a large file.", "Choose file", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            End If
        End Using
 
        ' Set the default scrollposition of RTB's
        SendMessage(RTB1.Handle, EM_GETSCROLLPOS, 0, ptScrollPos_RTB1)
        SendMessage(RTB2.Handle, EM_GETSCROLLPOS, 0, ptScrollPos_RTB2)
    End Sub
 
    Private Sub RTB1_VScroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RTB1.VScroll
        ' Do not continue if the scroll was made by this program and not by the user (to prevent endless loops)
        If blnScrolled_RTB2 Then Exit Sub
 
        ' Set a flag so RTB2 knows it is scrolled by RTB1 and not by the user
        blnScrolled_RTB1 = True
 
        Dim diff As Integer
 
        ' Check scrollposition after scrolling
        Dim pt As Point
        SendMessage(RTB1.Handle, EM_GETSCROLLPOS, 0, pt)
 
        ' Compare Y-value (height) with scrollposition before scrolling
        diff = pt.Y - ptScrollPos_RTB1.Y
 
        ' Set RTB1 scrollposition variable to new scrollposition
        ptScrollPos_RTB1 = pt
 
        ' Only sync scrolling when checkbox is checked
        If chk.Checked Then
            ' Get RTB2 scrollposition
            Dim pt2 As Point
            SendMessage(RTB2.Handle, EM_GETSCROLLPOS, 0, pt2)
 
            ' Set RTB2 scrollpos Y (height) to old scrollposition + the difference in RTB1 scrollposition
            SendMessage(RTB2.Handle, EM_SETSCROLLPOS, 0, New Point(pt.X, diff + pt2.Y))
        End If
 
        ' Re-Set the flag
        blnScrolled_RTB1 = False
    End Sub
 
    Private Sub RTB2_VScroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RTB2.VScroll
        ' Do not continue if the scroll was made by this program and not by the user (to prevent endless loops)
        If blnScrolled_RTB1 Then Exit Sub
 
        ' Set a flag so RTB1 knows it is scrolled by RTB2 and not by the user
        blnScrolled_RTB2 = True
 
        Dim diff As Integer
 
        ' Check scrollposition after scrolling
        Dim pt As Point
        SendMessage(RTB2.Handle, EM_GETSCROLLPOS, 0, pt)
 
        ' Compare Y-value (height) with scrollposition before scrolling
        diff = pt.Y - ptScrollPos_RTB2.Y
 
        ' Set RTB2 scrollposition variable to new scrollposition
        ptScrollPos_RTB2 = pt
 
        ' Only sync scrolling if checkbox is checked
        If chk.Checked Then
            ' Get RTB1 scrollposition
            Dim pt2 As Point
            SendMessage(RTB1.Handle, EM_GETSCROLLPOS, 0, pt2)
 
            ' Set RTB2 scrollpos Y (height) to old scrollposition + the difference in RTB2 scrollposition
            SendMessage(RTB1.Handle, EM_SETSCROLLPOS, 0, New Point(pt.X, diff + pt2.Y))
        End If
 
        ' Re-Set the flag
        blnScrolled_RTB2 = False
    End Sub
End Class