This is a known problem in the controls using Popup (RadMenu, RadComboBox, RadDatePicker) and RadWindow. In order to be able to stack windows one over another, RadWindow sets Z-Index on its own Popup. Since the popups of the other controls have no Z-Index, they appear below RadWindow. The problem doesn't appear the first time, because the first open RadWindow has no Z-Index.
We are currently testing a workaround, which most probably will be included in the next release of RadControls for Silverlight. However, it has some limitations - the standard Tooltip control is displayed below all RadControls which use Popup :)
This is a common problem for controls which display popups - our ASP.NET AJAX controls have it too, but in ASP.NET AJAX we have much more control over the AJAX framework and we can avoid it without much interference to other controls. I hope that the official release of Silverlight 2 will provide enough possibilities for customizing the standard controls, hence more ways to workaround such problems.
The bug report has been logged, you will receive an email, containing more details about our Early Adoption Program
Valeri Hristov (Silverlight Team)
the Telerik team
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