Hi Andy Schuler,
Thanks for contacting Syncfusion support.
We have checked your query about how to restrict Accordion to occupy whole space while expanding TreeViewAdv. By default, Height of the AccordionItem will be updated based on the height of its content. Here TreeViewAdv is the content of AccordionItem, if we expand treeviewitem height of the TreeViewAdv will get changed. So the accordion item will get arranged based on the height of TreeViewAdv. So we suggest you to set height for the TreeView in order to enable scrollbar for the TreeView. We have prepared sample for the same. In this sample we have set 60 as height of the TreeViewAdv. By doing this, ScrollViewer will get enabled while expanding TreeViewItem.
If we have misunderstood your query please get back to us with more details about your requirement. This would help us to provide solution at earliest.
Regards,
Durga S.