When I moved this into ControlCore, I forgot that UserScrollViewport is usually triggered by the scrollbar updating, so it doesn't ask the UI to update. Since this logic is in ControlCore, it's sorta in a weird place where it needs to communicate both up and down:
* update the `Terminal`'s viewport position
* update the `TermControl`'s scrollbar position

* [x] Closes a bug bash bug

Co-authored-by: Carlos Zamora <carlos.zamora@microsoft.com>