The ScrollBar Template features some built-in Setters that come included with the Element. However, unlike most other Templates we provide, it appears that the Scrollbar's Setter's conditions are unchangeable.

See attached video covering Repro Steps until Step 7).
(Steps 7) to 9) were discovered after capturing the video)

Note that is potentially/likely related to the Auto-Crash Report pertaining to setting a new custom Orientation: Horizontal Setter within a Scrollbar Template crashing Studio. (Possible #1061)

Repro Steps:

1) Create a new Project featuring a MainPage.xaml
2) Introduce a ScrollViewer Element to the Page
3) Right-Click on the ScrollViewer Element, and create a Template saved to GlobalResources.
4) Within the ScrollViewer Template, identify the VerticalScrollBar Element, and create a new Template from it.

5) From within the ScrollBar's Template, navigate to its Interactivity/Setters Panel, and select its built-in Setter.
6) Attempt to edit the Setter's 'Value' field from 'Vertical' to 'Horizontal'.
Observe how any changes to the Value systematically revert it back to 'Vertical'.

7) Attempt to edit the Setter's 'Property' field to Opacity.
Observe how in the Interactivity/Setters Panel, the readout has correctly changed to 'Opacity'.

8) Attempt to edit the Setter's 'Value' field from 'Vertical' to '0'. (which should be a valid Opacity Value)
Observe the Value continues to revert to 'Vertical' even if we are not even in an 'Orientation' Property anymore.

9) Edit the Setter's 'Property' field back to 'Orientation'.
Observe how in the Interactivity/Setters Panel, the readout is still displaying "Opacity".

Expected Results:
Setters can be freely, and successfully changed within Templates.