When I place a TextBlock inside of a Horizontally Aligned StackPanel it does not wrap. I realize that this is because the available width of the StackPanel is PositiveInfini
If you were like me and was wondering why the Textblock was still not wrapping after defining a Width.
Try adding a MaxWidth to the Textblock with same same value as Width,
This stops the control from growing in Width once the application has started.
Hope this helps!