Hello Greg Duffield,
Currently you can only select from the preset controls and you can not select additional classes like ScrollViewerExtensions. It may sound confusing, but if you select RadTreeView or RadPanelBar from the Navigation assembly, you'll get the ScrollViewerExtensions from Controls automatically. (because these controls use them internally. You should also upload the Controls dll). This is one solution for now.
Do you think it will be ok for you, if we provide a list with all the extensions (under an "Extensions node"), so you will be able to choose some? We will do this for the upcoming official release of the Minifier.
Thank you for any feedback.
the Telerik team
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