Well, it seems there is a simple solution.
I went through all the posts here on this Forum and used my trusted source Google.
I host the Html Placeholder in a ScrollView.
<ScrollViewer Height="122" HorizontalAlignment="Left" Margin="41,66,0,0"
Name="scrollStoryboard" VerticalAlignment="Top" Width="514">
<telerik:RadHtmlPlaceholder Name="radHtmlStoryboard" />
Then on OnNavigatedTo event of my page I do a quick hide and show on the Placeholder and set the scolling and overflow properties. I should be able to set these properties on the OnLoad event but it does not work.
radHtmlStoryboard.Visibility = System.Windows.Visibility.Collapsed;
radHtmlStoryboard.Visibility = System.Windows.Visibility.Visible;
(radHtmlStoryboard.HtmlPresenter.Children as HtmlElement).SetAttribute("scrolling", "no");
(radHtmlStoryboard.HtmlPresenter.Children as HtmlElement).SetAttribute("overflow", "visible");
And whenever I write something to the HtmlSource, I call the UpdateLayout followed by the ScrollToVerticalOffset on the ScrollView
htmlText = transcript .MessageText + "<br/>";
radHtmlStoryboard.HtmlSource += htmlText;
This works like a charm and scrolls to the bottom every time. The overflow property is needed if the output is too wide for the HtmlPlaceholder and forces a wrap on the HtmlPlaceholder and then a vertical scrollbar would appear.