RadGridView is a virtualized control and as such, its visual elements are recycled and reused on scrolling. You cannot get one instance of GridViewRow and place it in a different place. The only thing is to control the order of the items in which they are displayed in the grid.
One possible way to go is to created pinned rows as illustrated in the "Frozen / Pinned rows in RadGridView for Silverlight
" blog post. However, please keep in mind that this is just a showcase on how to do something and it is not something we support internally.
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