Hi Jerome,
Thank you for your interest in Syncfusion products.
I am afraid that I am unable to see the mentioned issue of hiding and showing the rows by setting the row heights individually, does not misbehaves on the scroll bar and it updates the scroll bar as expected.
I have attached the sample in the below location, which is a manual relation sample with hierarchical view, added a button to show and hide the row:
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=ShowHideRows-434207411.zipPlease try to reproduce the issue in the sample and update us with a modified sample or a new minimal sample which reproduce the issue. Let me know if you have any queries.
Regards,
Nirmal