Generally speaking, when a RadWindow is opened, it does not show the content page until it is fully loaded (e.g. window.onload is fired for the content page). During that time the IFRAME that contains the content page is with dimensions 0 by 0 pixels. Once the page is loaded, the IFRAME is expanded to fit the RadWindow.
This behavior is controlled by the ShowContentDuringLoad property. If it is set to true, RadWindow will act exactly like an IFRAME and will show the content page immediately after opening. Now if your tabstrip control is positioned depending on the size of the content page, when ShowContentDuringLoad is set to false, the tabstrip will use the 0 by 0 pixels dimensions which will result in a problem in the positioning.
Can you please let me know why you need to set the ShowContentDuringLoad property to false?
the Telerik team