How to set width to 100% in WPF
It is the container of the Grid that is imposing on its width. In this case, that's a ListBoxItem, which is left-aligned by default. You can set it to stretch as follows:
<ListBox> <!-- other XAML omitted, you just need to add the following bit --> <ListBox.ItemContainerStyle> <Style TargetType="ListBoxItem"> <Setter Property="HorizontalAlignment" Value="Stretch"/> </Style> </ListBox.ItemContainerStyle></ListBox>