Synched RichTextBoxes, Numbered, 3 Jan 2018 - VB.NET

I've been playing with this all day, and I finally got it work perfectly. NickThissen's post on synchronizing richtextboxes got me started, but I slimmed it down, adding numbered lines.

Attachment 154987

There are no editing functions except for pasting text into it by pressing Ctrl-P besides typing.

RichTextBox1's backcolor is set to "Control" in properties, borderstyle "None", scrollbars "None", readonly "True", docked "Left", with a width of 70.

RichTextbox2's backcolor is "Window", borderstyle "None", wordwrap "False", docked "Fill".

Enjoy! :)

Code:

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Public Class Form1

    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


    Private Sub Form1_Load_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        RichTextBox1.SelectionAlignment = HorizontalAlignment.Right
        RichTextBox1.Text = "1"
        RichTextBox1.Enabled = False
    End Sub


    Private Sub RichTextBox2_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles RichTextBox2.KeyUp
        If (e.KeyCode = Keys.P AndAlso e.Modifiers = Keys.Control) Then
            RichTextBox2.Text = Clipboard.GetText()
            RichTextBox2.SelectAll()
            RichTextBox2.SelectionFont = New Font("Microsoft Sans Serif", 10, FontStyle.Regular)
            RichTextBox1.SelectAll()
            RichTextBox1.SelectionFont = New Font("Microsoft Sans Serif", 10, FontStyle.Regular)
        End If
    End Sub


    Private Sub RichTextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RichTextBox2.TextChanged
        Dim count As Integer
        Dim count2 As String
        Dim myList As List(Of String) = RichTextBox1.Lines.ToList()
        Dim minus As Integer
        Dim holder As String

        count = RichTextBox2.Lines.Length
        holder = CStr(RichTextBox1.Lines.Length)
        count2 = " "

        If RichTextBox1.Lines.Length > count Then
            For minus = 1 To CInt(CDbl(holder) - count)
                If myList.Count > count Then
                    myList.RemoveAt(myList.Count - 1)
                    RichTextBox1.Lines = myList.ToArray()
                    RichTextBox1.Refresh()
                End If
            Next
        Else
            If count = RichTextBox1.Lines.Length Then
                count = RichTextBox1.Lines.Length
            End If
        End If

        If count > RichTextBox1.Lines.Length + 1 Then
            count2 = CStr(count - RichTextBox1.Lines.Length)
            For minus = 1 To CInt(count2)
                RichTextBox1.AppendText(Environment.NewLine & minus + count - CDbl(count2))
            Next
        End If

        If RichTextBox1.Text.Length = 0 Then
            RichTextBox1.Text = "1"
        Else
        End If

        If RichTextBox1.Lines.Length < count Then
            RichTextBox1.AppendText(Environment.NewLine & count)
        Else
        End If

        If RichTextBox1.Text.Length = 2 Then
            RichTextBox1.AppendText("2")
        Else
        End If

        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


    Private Sub RichTextBox2_VScroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox2.VScroll
        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


    Private Sub Form1_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Resize
        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


End Class

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Coming feature...
Document map column, for fast navigation of large documents in a richtextbox, by hovering over an area of a captured thumbnail of it like in Notepad++.

The zipped project below uses an image divider separating the richtextboxes.

Edited this comment to remove old code.