I have tested the provided code, but everything works as expected at our side - the group header is shown with Xaml and NoXaml assemblies. Could you try to create a sample app demonstrating the erroneous behavior and send it to us? You should open a support ticket and attache it there.
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I am able to use a CollectionViewSource to group my listbox successfully. However, as soon as I try to set the template of the header, the text disappears, regardless of what color/size/weight of the font.
The header is still present, because the space is there and when I change the font size the gap for the group header changes. But as long as I have a header template set, I am unable to actually see the ListBox group header.
Dario, if you were able to resolve this issue, would you mind sharing your solution with me?
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