I'm opening up several RadWindows in my Silverlight application that contain RadHtmlPlaceholders and I noticed some strange behavior (see attached screen shots).
The issues you are observing is a result of a limitation of the HTMLPlaceholder control that is using an IFrame that is displayed on the top of the Silverlight. Because of that it is even over the RadWindow control. Unfortunately there is not way to fix this as the IFrames of the HTMLPlaceholder control are always over the Silverlight.