test.cs

private bool _isMarkupVisible = true;

private void btnToggleMarkup_Click(object sender, EventArgs e) {
  _isMarkupVisible = !_isMarkupVisible;
  ToggleMarkup();
}

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

private void ToggleMarkup() {
  foreach (TXTextControl.TrackedChange change in textControl1.TrackedChanges) {

    if (change.ChangeKind == TXTextControl.ChangeKind.DeletedText)
      continue;

    change.HighlightMode = _isMarkupVisible == true ?
      TXTextControl.HighlightMode.Always : 
      TXTextControl.HighlightMode.Never;
  }
}