The issue you are experiencing is very strange, because RadOutlookBar's ControlTemplate does not have a ScrollViewer for wrapping its selected item's content. The ScrollViewer you fail to see is probably the one of the TreeView. Is there a chance you can provide a sample project reproducing the issue (you will need to open a support ticket in order to attach a project)? This way I can better investigate the issue, because currently I wasn't able to reproduce it.
the Telerik team