By default, RadWindow is centered after autosizing, but you have modified its AutoSizeBehaviors so that its top left corner stays in place. Removing that will have it center itself after autosizing. Thus, calling center() immediately after autosizing is not needed.
On scrolling - the Pin behavior of the RadWindow should be used for keeping it in he same position relative to the viewport instead of overriding its position to fixed. The following function can make sure that it is always shown pinned:
which is, of course, attached to the OnClientShow event of the control.
I am attaching here your page with my modifications and comments next to them as a reference. A flash video with the expected behavior shown in my end is also present in the archive.
I also advise that you examine the following help article on configuring autosizing: http://www.telerik.com/help/aspnet-ajax/window-troubleshooting-autosize-not-correct.html
the Telerik team
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