I am posting this in case someone else needs it.
From my original ticket the first time I had this issue
Posted on on Apr 26, 2011
I have a page that I am not sure what is causing the Property Editor for the Widgets to not display properly. I have removed everything I know to remove (AjaxManager, ScriptManager, RadDockLayout, RadWindowManager, etc). When I edit the page, I can drop widgets on to the page but when I click edit it shows a blank window see image1.
After further investigation, I looked at the css and found the style property for the iframe has top: -10000px. (screenshot 2) If i change that to top: 0px; then it displays fine. (screenshot 3).
This happens with any widget even the default content editor widget.
I found that in my stylesheets I had the following style
removing position: relative fixes the problem.