■No35280に返信(ま〜さんの記事)
> こんにちは、体調を崩して回答が遅れました。すみません。

お大事に。

> ですがTabPage環境化ではズレました。
> Parentをと思って色々やってみましたが中々上手くいかずTabPage下の座標知る方法はないのでしょうか？

むむむ、やっかいですね高DPI。
スクリーン座標を取るのが目的でないなら、以下のコードでキャプチャはできると思います。

Imports System.Runtime.InteropServices

Public Class Form1

    Public Sub New()
        InitializeComponent()
        PictureBox1.SizeMode = PictureBoxSizeMode.Zoom
        PictureBox1.Size = RichTextBox1.Size
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim bmp = CreateBitmapFromControl(RichTextBox1)
        If PictureBox1.Image IsNot Nothing Then
            PictureBox1.Image.Dispose()
        End If
        PictureBox1.Image = bmp
    End Sub

    Private Const SRCCOPY As Integer = &HCC0020

    <DllImport("User32.dll")>
    Private Shared Function GetWindowDC(hWnd As IntPtr) As IntPtr
    End Function

    <DllImport("user32.dll")>
    Private Shared Function ReleaseDC(hWnd As IntPtr, hDC As IntPtr) As Boolean
    End Function

    <DllImport("gdi32.dll")>
    Private Shared Function BitBlt(hdcDest As IntPtr,
                                   nXDest As Integer, nYDest As Integer,
                                   nWidth As Integer, nHeight As Integer,
                                   hdcSrc As IntPtr,
                                   nXSrc As Integer, nYSrc As Integer,
                                   dwRop As Integer) As Boolean
    End Function

    Private Shared Function CreateBitmapFromControl(con As Control) As Bitmap
        Dim cs = con.Size
        Dim bmp As New Bitmap(cs.Width, cs.Height)
        Using g = Graphics.FromImage(bmp)
            Dim hdcDest = g.GetHdc()
            Dim hdcSrc = GetWindowDC(con.Handle)
            BitBlt(hdcDest, 0, 0, cs.Width, cs.Height, hdcSrc, 0, 0, SRCCOPY)
            ReleaseDC(con.Handle, hdcSrc)
            g.ReleaseHdc()
        End Using
        Return bmp
    End Function

End Class