How to animate autosized element?

Hi, I have a treeview with a styled items. I would like them to appear smoothly, like it was in MSN messenger. The first problem is in case of style I can select it only like single object and cant select child to animate it. The second problem is more important - I cant animate this item, because it size is AutoSize and I get error "You can not animate autosized element, first, make it fixed size". But I cant make it fixed size, because item size depend on it content.

How to solve this problem?

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Aleksey