test.cs

private bool scrolling = false;

private void setScrollOffset() {
  // remember old scroll location
  var oldScrollLocation = textControl1.ScrollLocation;
  // scroll to bottom to get max value
  textControl1.ScrollLocation = new Point(0, int.MaxValue);
  int scrollY = textControl1.ScrollLocation.Y;

  // mirror the max value in custom scroll bar
  vScrollBar1.Maximum = scrollY;

  // restore scroll location
  textControl1.ScrollLocation = oldScrollLocation;
}

private void textControl1_Changed(object sender, EventArgs e) {
  setScrollOffset();
}

private void textControl1_VScroll(object sender, EventArgs e) {
  if (scrolling == true)
    return;

  // set the scroll location
  if (textControl1.ScrollLocation.Y <= vScrollBar1.Maximum) {
    vScrollBar1.Value = textControl1.ScrollLocation.Y;
    vScrollBar1.SmallChange = 300; // fine tune speed here
    vScrollBar1.LargeChange = 300;
  }
}

private void vScrollBar1_Scroll(object sender, ScrollEventArgs e) {
  // mirror the scroll location
  scrolling = true;
  textControl1.ScrollLocation = new Point(0, e.NewValue);
  scrolling = false;
}