Keep in mind that the RadHtmlPlaceHolder control is not placed directly in the Silverlight plugin, which means that it is positioned above the plugin. When you place a html placeholder inside a ViewBox it won't be resized correctly. This is why instead of ViewBox I recommend you to use a Grid with columns and rows, this way the the placeholder will be resized properly. Another approach could be to handle the SizeChanged event of the viewbox and set the width and height of the placeholder inside the event handler. Here is an example code:
sender, SizeChangedEventArgs e)
Let me know if you have further questions.
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