Richtextbox control does not respond to touchscreen controls for scrolling and only highlights text instead

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### Bug explanation

I built a modern log interface for legacy chat software and use MaterialDesign to theme everything. I'm using the MaterialDesign Richtextbox style and when I try and enable the following:

```
ScrollViewer.CanContentScroll="True"
ScrollViewer.PanningMode="VerticalFirst"
Stylus.IsPressAndHoldEnabled="False"
Stylus.IsFlicksEnabled="False"
```

It still only highlights the text in the Richtextbox control when using touchscreen controls. Does this mean that I need to override the control template or is there something that I'm missing for this?

I've considered making changes to the MaterialDesign.Textbox.xaml prior to compile and adding the `CanContentScroll=True` there but not sure if that's going to fix it or not.

Does anyone have any suggestions for this? I have a Surface Pro that I'm testing on and I've had reports of it not working for others as well.

Please let me know if you need any further details or code, I'm happy to share.

Thanks!

### Version

5.1.0.0

Start by reproducing the touchscreen behavior with the MaterialDesign Richtextbox style on a Surface Pro or equivalent WPF setup. Inspect MaterialDesign.Textbox.xaml and the control template to determine whether the configured ScrollViewer and stylus properties reach the Richtextbox. Done means vertical touch scrolling works without selecting text, with the result verified on the reported version or a current build.

Written by the indexing model from the issue text.