Currently, if within a VerticalScrollBar Template, the Thumb Element's Width value cannot be changed from the default value of "10".
It also cannot be successfully converted to a Local Value.

See attached video.

Note: Although it is not possible to change the Thumb's value within the ScrollBar, it is however possible to adjust it within the Thumb's Template. However, as the Width of the Thumb's Border within its Template is set to "Auto", it should in theory be overridden by the value set at the VerticalScrollBar level.

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) 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.

4) Identify the PART_VerticalScrollBar and create a new Template for it. (Saved to GlobalResources, saved as any name)
This will open the inline editor for the ScrollViewer/PART_VerticalScrollBar Template.

5) In the PART_VerticalScrollBar Template, identify the Thumb Element (Not the Thumb Parent), and select it.

6) In the Thumb Element's Properties Panel, change the width to any desired value other than the default 10"".

Observe how the value constantly reverts back to "10" no matter what value input.

7) From the Thumb's Width Property, select "Convert to Local Value" from its 3-Dot menu.

Observe how no changes are observed.

Expected Results: The Thumb's Width Value can be freely modified at the ScrollBar Template level.