Why is a ListBoxItem not calling MeasureOverride when its width is changed?

Question

Ok, for illustrative purposes, below I created a subclass of ListBoxItem and a subclass of ListBox which uses it as its container by overriding both IsItemItsOwnContainerOverride and GetContainerForItemOverride.

Now when the window first appears, as expected, MeasureOverride is called on every ListBoxItem (with Infinity,Infinity) followed by ArrangeOverride being called on every item.

However, when resizing the ListBox, only ArrangeOverride is called on the ListBoxItem, not MeasureOverride even though the metadata for the width property is set to AffectsMeasure.

NotE: I know I can get around this by setting ScrollViewer.HorizontalScrollbarVisibility to 'Disabled' in which case MeasureOverride does get called as expected because that scroll setting forces the items to match the width of the listbox and thus naturally would re-fire. However, I'm still trying to figure out why Measure isn't called by default anyway because the metadata for the Width property has the AffectsMeasure flag set and the width is changing via the ArrangeOverride step.

Is that flag just a hint for its container and in the case of a control placed in a ScrollViewer it's ignored? My guess is that unless you disable the scrolling, the controls have an infinite area available to them, so once they are measured, there's no need to re-measure them again. Disable the horizontal scrolling however and you're stating the width isn't unlimited, hence the MeasureOverride is called again. But that's just a guess, albeit a logical one.

Here's example code to play with. Create a new WPF project and paste this in the window's CodeBehind and look at the debug output. Next, set the HorizontalScrollbarVisibility flag and you'll see that it does get called.

public partial class MainWindow : Window
{
    public MainWindow(){
        InitializeComponent();
        var items = new List<object>(){ "This is a really, really, really, really long sentence"};
        var lbx = new ListBoxEx { ItemsSource = items };
        this.Content = lbx;
    }

}

public class ListBoxEx : ListBox
{
    protected override bool IsItemItsOwnContainerOverride(object item){
        return (item is ListBoxItemEx);
    }

    protected override DependencyObject GetContainerForItemOverride(){
        return new ListBoxItemEx();
    }
}

public class ListBoxItemEx : ListBoxItem
{
    protected override Size MeasureOverride(Size availableSize){
        Console.WriteLine("MeasureOverride called with " + availableSize);
        return base.MeasureOverride(availableSize);
    }

    protected override Size ArrangeOverride(Size finalSize){
        Console.WriteLine("ArrangeOverride called with " + finalSize);
        return base.ArrangeOverride(finalSize);
    }

}

Answer 1

Ok, I think the answer is it's not firing because the Measure pass is affecting the panel, not the items, and the panel itself hasn't resized so there's no reason to re-measure its children.

However, when setting ScrollViewer.HorizontalScrollbarVisibility to Disabled, the width of the panel tracks the width of the ListBox, not its contents, therefore the children do need to be re-measured, hence that's why they were in that case.