TreeView set to OutlookExpress with simple 3 column dataset with 19 rows. As you can see in the picture as you start to collapse the screen the scrollbar appears about a row and a half late.
Sample app included.
Thank you for contacting Infragistics!
I am able to reproduce the behavior you described. I have submitted this development issue (264042) for further investigation and have created a private case (CAS-202419-V2M1X6) to link this issue.
You can see this private case here: https://www.infragistics.com/my-account/support-activity
Please let me know if I can provide you further assistance.