Scrollbar inconsistently handles clicking on scrollbar region (not thumb)

### Describe the bug

From https://github.com/microsoft/terminal/issues/18063
> Typically when mouse click the scrollbar(not the thumb), hold the mouse button down. The scrollbar will:
page up, , page up, page up, page up, page up,...
Then thumb will stop at the mouse pointer position.
> Instead, there are 3 random behaviors.
> 1. Page up/down once and stop.
> 2. Thumb not stop. And reach the top/bottom position.
> 3. Thumb move across the mouse pointer position.

https://github.com/user-attachments/assets/12989340-50c2-4ff1-a14c-82d142296614

### Steps to reproduce the bug

1. Go to a page with a long scroll region in the WinUI 2 gallery (I used ListView)
2. Click and hold on the scrollbar region (not the thumb)

### Expected behavior

We should be consistent. I'd expect it to scroll to the pointer position.

### Screenshots

_No response_

### NuGet package version

WinUI2. Also repros in WinUI3

### Windows version

_No response_

### Additional context

_No response_

Reproduce the behavior in the WinUI 2 Gallery using a ListView with a long scroll region, then compare it with WinUI 3 as described. Investigate the scrollbar-region click and hold behavior; done means clicking outside the thumb consistently moves the thumb to the pointer position rather than producing any of the three reported outcomes.

Written by the indexing model from the issue text.