By default, ItemsPanelTemplate is a stack panel, as far as I remember, this means that it will not automatically lead to the behavior of children. You will need to change it, I will edit it in a second on my mobile device, waiting for my laptop to load :)
Edit:
Do not ignore what I was thinking about ItemsControl , JBrooks is right, just make sure that HorizontalContentAlignment set to Stretch , because by default it is Left .
<ListView Name="ResultsView" HorizontalContentAlignment="Stretch">
This should work fine - all controls that use XXXXContentAlignment default to Left , which, in my opinion, is a bit contrary to other controls (like Grid )
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