I just answered your question in the support ticket that you sent us. For convenience I am pasting part of my reply below:
By design, when a RadWindow is closed, it's object is not destroyed - the RadWindow remains on the page but it is hidden. This is done to provide caching features for the control so there would be no unnecessary round-trips to the server for loading the content page. Of course there are scenarios when this is not convenient - when RadWindow is used for a dialog for example. That is why we added the ReloadOnShow feature that reloads the content page when the RadWindow is shown.
This feature however, leads to the behavior that you observe - when you call the window for a second time, you see for a brief time the cached content. This, however is needed because if we first load the Url in the window and then show it, a code in the content page that tries to set focus for example will break because the container is hidden. That is why we have the ShowContentDuringLoad property - it is fired at the moment when the RadWindow is shown and displays a loading animation until the content page finishes loading, e.g. window.onload is fired in the content.
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