StackPanel никак не прикрепляется к левому краю.
<TabItem Header="Ассортимент">
<Grid>
<ListBox VerticalAlignment="Top" ItemsSource="{Binding Source={StaticResource assortmentCollection}}"
HorizontalAlignment="Stretch" AlternationCount="2">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="auto"/>
</Grid.ColumnDefinitions>
<StackPanel>
<TextBlock Padding="5" FontSize="20" FontWeight="Bold" Text="{Binding Name}"/>
<StackPanel Margin="5,0,0,0" Orientation="Horizontal">
<TextBlock FontSize="14" FontWeight="Bold" Text="Калорийность "/>
<TextBlock FontSize="14" Text="{Binding Calorific}"/>
<TextBlock FontSize="14" Text=" ккал."/>
</StackPanel>
<TextBlock Padding="5" FontSize="14" Text="{Binding Description}"/>
</StackPanel>
<StackPanel Grid.Column="1" Orientation="Horizontal" HorizontalAlignment="Left">
<StackPanel VerticalAlignment="Center" Orientation="Horizontal" Margin="5">
<TextBlock FontSize="20" FontWeight="Bold" Text="Цена: "/>
<TextBlock FontSize="20" Text="{Binding Price, StringFormat=N2}"/>
<TextBlock FontSize="20" Text="₽" Margin="1"/>
</StackPanel>
<Button Content="Добавить" Height="42" Padding="6" Command="{Binding AddToCart, Source={StaticResource cart}}"
CommandParameter="{Binding}" Background="Transparent" BorderThickness="2" HorizontalAlignment="Right">
</Button>
</StackPanel>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
</TabItem>
В соседней вкладке почти такой же код, но всё работает
<TabItem Header="Корзина">
<Grid DataContext="{StaticResource cart}">
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="60"/>
</Grid.RowDefinitions>
<ListBox VerticalAlignment="Top" ItemsSource="{Binding Path=CartItems, Mode=OneWay, Source={StaticResource cart}}"
HorizontalContentAlignment="Stretch" AlternationCount="2">
<ListBox.ItemTemplate>
<DataTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="auto"/>
</Grid.ColumnDefinitions>
<StackPanel>
<TextBlock Padding="4" FontSize="20" FontWeight="Bold" Text="{Binding Product.Name}"/>
<StackPanel Margin="4" Orientation="Horizontal">
<TextBlock FontSize="12" FontWeight="Bold" Text="Цена товара: "/>
<TextBlock FontSize="12" Text="{Binding Product.Price, StringFormat=N2}"/>
<TextBlock FontSize="12" Text="₽"/>
</StackPanel>
<StackPanel Margin="4" Orientation="Horizontal">
<TextBlock FontSize="12" FontWeight="Bold" Text="Количество: "/>
<TextBlock FontSize="12" Text="{Binding Quantity}"/>
<TextBlock FontSize="12" Text="шт."/>
</StackPanel>
<StackPanel Margin="4" Orientation="Horizontal">
<TextBlock FontSize="15" FontWeight="Bold" Text="Всего за позицию: "/>
<TextBlock FontSize="15" Text="{Binding Sum, StringFormat=N2}"/>
<TextBlock FontSize="15" Text="₽"/>
</StackPanel>
</StackPanel>
<StackPanel Grid.Column="2">
<Button Content="Увеличить" Margin="3" FontSize="14" Padding="7" Command="{Binding AddQuantity, Source={StaticResource cart}}"
CommandParameter="{Binding}" Background="Transparent" BorderThickness="2">
</Button>
<Button Content="Уменьшить" Margin="3" FontSize="14" Padding="7" Command="{Binding MinusQuantity, Source={StaticResource cart}}"
CommandParameter="{Binding}" Background="Transparent" BorderThickness="2">
</Button>
<Button Content="Удалить" Margin="3" FontSize="14" Padding="7" Command="{Binding RemoveProduct, Source={StaticResource cart}}"
CommandParameter="{Binding}" Background="Transparent" BorderThickness="2"/>
</StackPanel>
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
<StackPanel Grid.Row="1" Orientation="Horizontal" Background="#D9F066">
<TextBlock Padding="5" VerticalAlignment="Center" FontSize="18" FontWeight="Bold" Text="ИТОГО:"/>
<TextBlock Padding="5" VerticalAlignment="Center" FontSize="18" FontWeight="Bold" Text="{Binding TotalPrice, StringFormat=N2}"/>
<TextBlock Padding="5" FontSize="18" FontWeight="Bold" VerticalAlignment="Center" Text="₽"/>
<Button Margin="5" Width="100" Height="50" FontSize="18"
Content="Оплата" HorizontalContentAlignment="Center"
Command="{Binding Checkout}" Background="#e2ebda"/>
</StackPanel>
</Grid>
</TabItem>
