Multiline Textbox with colored lines per group (criteria for grouping is the data itself...)

Thanks to one of the experts here,  I was able to build an extension to the textbox that allows me to color the rows of a multicolor textbox.

Now the problem is that if I scroll the textbox vertically,  the grouping of colored row "sets" shifts since the code  uses the row position of the visible rows in the textbox.

I think what I need is to know at any point in time which rows within a set of rows is visible and to color them according to where they are in the entire textbox's rows (using the line data for criteria).

If anyone can tell me how to determine the first and last visible row in the textbox,  I think I will be able to put this problem to rest.

I am including my textbox class for review.  There are two routines of particular interest here.  The original drawRect (drawRectOld) and the new drawRect that uses some row criteria to decide when to switch colors.

Thanks,
Howard

Public Class ColorfulTextbox
    Inherits TextBox
 
    Private Const WM_PAINT As Integer = &HF
    Private Const WM_VSCROLL As Integer = &H115
 
    Protected Overrides Sub OnTextChanged(ByVal e As System.EventArgs)
 
        MyBase.OnTextChanged(e)
        Me.Invalidate()
 
    End Sub
 
    Protected Overrides Sub WndProc(ByRef wndmsg As System.Windows.Forms.Message)
 
        MyBase.WndProc(wndmsg)
 
        Select Case wndmsg.Msg
            Case WM_PAINT
                drawRect()
 
            Case WM_VSCROLL
                Me.Invalidate()
 
        End Select
 
    End Sub
 
    Sub drawRect()
        Dim nRowHeight As Integer = 16
        Dim rectF As System.Drawing.RectangleF
        Dim nRows As Long = Me.Height / nRowHeight + 1
        Dim nlp As Long
 
        Dim clr As Color
        clr = Color.Bisque
        clr = Color.FromArgb(clr.A / 2, clr.R, clr.G, clr.B)
 
        Dim clearIt As Color = Color.White
        clearIt = Color.FromArgb(clearIt.A / 2, clearIt.R, clearIt.G, clearIt.B)
        Dim ClearBrush As New SolidBrush(clearIt)
 
        Dim brush As New SolidBrush(clr)
 
        Dim g As Graphics = Me.CreateGraphics()
        Dim arLines As String() = Me.Text.Split(vbCrLf)
 
        If UBound(arLines) = 0 Then Exit Sub
 
        Dim bColoringGroup As Boolean = True
        Dim bGroupChange As Boolean = False
 
        rectF = New System.Drawing.RectangleF(0, 0, Me.Width, nRowHeight)
        g.FillRectangle(brush, rectF)
 
        For nlp = 1 To nRows - 1
 
            If arLines(nlp - 1).Split(" ")(0).Trim = arLines(nlp).Split(" ")(0).Trim Then
                bGroupChange = False
            Else
                bGroupChange = True
                bColoringGroup = Not bColoringGroup
            End If
 
            If bColoringGroup Then
                rectF = New System.Drawing.RectangleF(0, nlp * nRowHeight, Me.Width, nRowHeight)
                g.FillRectangle(brush, rectF)
            Else
                rectF = New System.Drawing.RectangleF(0, nlp * nRowHeight, Me.Width, nRowHeight)
                g.FillRectangle(ClearBrush, rectF)
            End If
 
        Next
 
        g.Dispose()
        brush.Dispose()
 
    End Sub
 
    Sub drawRectOld()
        Dim nRowHeight As Integer = 16
        Dim rectF As System.Drawing.RectangleF
        Dim nRows As Long = Me.Height / nRowHeight + 1
        Dim nlp As Long
        Dim clr As Color
        clr = Color.Bisque
 
        clr = Color.FromArgb(clr.A / 2, clr.R, clr.G, clr.B)
 
        Dim brush As New SolidBrush(clr)
 
        Dim g As Graphics = Me.CreateGraphics()
 
        For nlp = 0 To nRows - 1 Step 2
            rectF = New System.Drawing.RectangleF(0, nlp * nRowHeight, Me.Width, nRowHeight)
            g.FillRectangle(brush, rectF)
        Next
 
        g.Dispose()
        brush.Dispose()
 
    End Sub
 
End Class

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