I tried to reproduce your issue and it seems that the code works as expected on our side. Can you please take a look at the attached project and see if I missed something? Also, keep in mind that the RadHtmlPlaceholder
control depends on the style settings (overflow, scrolling attributes) of the page that hosts the Silverlight plug-in. Hence you can control the behavior using those settings.
Pavel R. Pavlov
the Telerik team