Border disappearing on ItemsControl resize (using “stretchpanel”)

Question

Yesterday I asked how to get an ItemsControl to have its children equally distributed across the available space; and after happily reading the answer, I wrote (well actually ki

Answer 1

It will work if you write your StretchPanel like this, which measure its child elements with zero width and height:

public class StretchPanel : Panel
{
    protected override Size MeasureOverride(Size availableSize)
    {
        foreach (UIElement element in Children)
        {
            element.Measure(new Size());
        }

        return new Size();
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        for (int i = 0; i < Children.Count; i++)
        {
            int x1 = (int)(finalSize.Width / Children.Count * i);
            int x2 = (int)(finalSize.Width / Children.Count * (i + 1));
            Children[i].Arrange(new Rect(x1, 0, x2 - x1, finalSize.Height));
        }

        return finalSize;
    }
}

and then set HorizontalAlignment="Stretch" (or don't set it at all, since that is the default value) on the ItemsControl.