ItemsControl items bindings called when collapsed - wpf

Question

I have an ItemsControl which displays a list of messages. It's defined as ...
<ItemsControl HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
ItemsSource="{Binding Messages}" >
</ItemsControl>
I then have a DataTemplate which handles the display for each message. It's defined as...
<DataTemplate DataType="{x:Type vm:MessageViewModel}">
<Button Command="{Binding CommandOpenPage}">
<Button.Template>
<ControlTemplate>
<Border Margin="2" BorderThickness="1"
BorderBrush="{Binding Flags, Converter={StaticResource msgFlagConverter}}"
Background="{Binding Flags, Converter={StaticResource msgFlagConverter}, ConverterParameter=1}" >
<TextBlock Text="{Binding Path=Message}" Style="{StaticResource ActionItem}" TextWrapping="Wrap" />
</Border>
</ControlTemplate>
</Button.Template>
</Button>
</DataTemplate>
Everything displays OK. My problem is when the parent controls are set to Visibility=Collapsed my ItemsControl still goes through the DataTemplate and calls the converters for BorderBrush and BackgroundBrush for each MessageViewModel.
This is bothersome because when the list is very large the bindings are set and converters are executed when they shouldn't. This list is only visible when the user chooses to see it. I understood the binding engine ignores elements under a collapsed parent. Is there an exception to this rule? Or am I just missing something?

Answer 1

I found my problem. The above ItemsControl and DataTemplate were in a UserControl. The visibility was originally handled inside the usercontrol itself by binding the main layout grid to a visibility property. By simply setting the user controls visibility in the parent XAML all bindings started behaving as expected.
This fixes my problem but I still don't understand the difference between setting the visibility of the main layout grid vs the visibility of the usercontrol itself.
<c:ApplicationMenuView Grid.Column="1" Grid.Row="4"
HorizontalAlignment="Left" Margin="1"
VerticalAlignment="Stretch"
DataContext="{Binding Menu}"
Visibility="{Binding IsVisible, Converter={StaticResource BooleanToVisibilityConverter}}"/>