There is no such event, but you can use OnClientNodeExpanding and OnClientNodeCollapsing event of the treeview to indicate the start of animation and OnClientNodeAnimationEnd to to indicate the end.
The other option is to use jquery resize
event like this:
$ = $telerik.$;
//do something here
"rtUL" is the element which is resized when a node is expanded or collapsed.
Hope this helps.
All the best,
the Telerik team
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