Here is one solution mostly based on XAML and databinding with the x:Bind extension (I've used a UserControl to be able to put the converter resource somewhere because with WinUI3 you can't put it under the Window element):

<UserControl
    x:Class="MyApp.UserControl1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:classes="using:MyApp.Models">
    <UserControl.Resources>
        <classes:VisibilityNegateConverter x:Key="vn" />
    </UserControl.Resources>

    <ComboBox x:Name="TheComboBox" ItemsSource="{x:Bind classes:FooBar.FooBars}">
        <ComboBox.ItemTemplate>
            <DataTemplate x:DataType="classes:FooBar">
                <StackPanel>
                    <TextBlock Text="{x:Bind Foo}" Visibility="{x:Bind TheComboBox.IsDropDownOpen}" />
                    <TextBlock Text="{x:Bind Bar}" Visibility="{x:Bind TheComboBox.IsDropDownOpen, Converter={StaticResource vn}}" />
                </StackPanel>
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>
</UserControl>

And the converter for the "reverse" visibility conversion between Boolean and Visibility ("forward" conversion is now implicit in WinUI3 and UPW for some times)

public class VisibilityNegateConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language) => (bool)value ? Visibility.Collapsed : Visibility.Visible;
    public object ConvertBack(object value, Type targetType, object parameter, string language) => throw new NotSupportedException();
}

Note: I've tried to use function binding to avoid the need for a converter, something like this:

<TextBlock Text="{x:Bind Bar}" Visibility="{x:Bind TheComboBox.IsDropDownOpen.Equals(x:False)}" />

But compilation fails miserably (maybe a bug in XAML compiler?)