Hi there, this PG shows that the mouse wheel does not work on the scrollviewer. But if I set the textblock resizeToFit to true, it works. So weird.
The best way I can explain it is how resizeToFit works.
if we turn on the bounding box we can see that without resizeToFix the TextBox will just end even though there is more text. But because the box is not bigger than the scroll viewer size there’s no reason to for the scroll viewer to scroll.
When we set resizeToFit to be true, the TextBox will scale to the proper size. Now it is bigger than the scroller viewer so we can start scrolling.
Hope that makes sense 
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In this way, I set the height of the textBox and scrollviewer. and it does work. Thanks!
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