The default ScrollBar Element includes 2 core parts:
The Thumb: The Part that can be clicked-and-dragged up and down to scroll a page up and down
and a pair of RepeatButtons, which allow the user to click into any area where the Thumb isn't present, to immediately jump the Thumb to the clicked position in the Scrollbar.

When dragging a Default ScrollBar Element into a Page, all Parts work as expected. However, when Creating a Template for the Scrollbar, only the Thumb portion of the functionality is conserved, and the two RepeatButtons lose all of their functionality.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) Introduce a ScrollViewer to the Page, and directly within it, drag in a TextBlock with a width of 80, and a Font Size of 300 as to make the ScrollViewer's ScrollBar visible once the next Step is completed.

3) Press Play, then click-and-drag the ScrollBar's Thumb up and down.

Observe how the content scrolls up and down.

4) Still in Play Mode, click into the empty regions of the Scrollbar where no Thumb is present.

Observe how the content Scrolls to the clicked position.

5) Exit Play mode and returnt to the Editor.

6) Create a new ScrollViewer Template from the ScrollViewer present on the stage. (Saved to GlobalResources, saved as any name)
This will open the inline editor for the ScrollViewer Template.

7) Press Play, then click-and-drag the ScrollBar's Thumb up and down.

Observe how the content scrolls up and down.

8) Still in Play Mode, click into the empty regions of the Scrollbar where no Thumb is present.

Observe how the functionality that was previously present, no longer functions.

Expected Results: Creating a Template conserves all functionality from the Default Element dragged into the Page.