This is an expected behavior.When you move an HTML object on a page and there is an iframe on that page, it is possible that the cursor go into the iframe. If this happens, the iframe will consume the cursor events, which will lead to problems with the resize / movement of the object on the main page. You can hide all iframes on the parent page when you move / resize RadWindow. I attached OnClientDragStart event of RadWindow which will disable this functionality.
resizeExtender = oWin._resizeExtender;
//DISABLE iframe hiding