ZoomContentControl: Infinitely calling MeasureOverride?

I'm implementing a ruler control (like found in most photo editors, text editors, etc).  In XAML, it's used like:

<local:Ruler ... >
   <Image Source="{Binding ...}" />
</local:Ruler>

 For its implementation, I derive from ContentControl, override OnRender() to draw the ruler itself, then position the child content like:

        protected override Size MeasureOverride(Size availableSize)
        {
            ContentPresenter childElement = (ContentPresenter)VisualTreeHelper.GetChild(this, 0);
            if (childElement != null) childElement.Measure(availableSize);
          //Console.WriteLine("Desired Size: " + childElement.DesiredSize.Width + ", " + childElement.DesiredSize.Height);
            return new Size(childElement.DesiredSize.Width, childElement.DesiredSize.Height + RulerHeight);
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            UIElement childElement = (UIElement)VisualTreeHelper.GetChild(this, 0);
            if (childElement != null) childElement.Arrange(new Rect(new Point(0.0, RulerHeight), finalSize));
            return finalSize;
        }

This seems to be working - both the ruler and the image are presented as expected (and of course, I can replace the Image with a StackPanel containing other elements, etc).

However, as soon as I try to wrap the image in a ZoomContentControl:

<local:Ruler ... >
  <actinav:ZoomContentControl IsPanPadVisible="False" IsResetViewButtonVisible="False">
    <Image Source="{Binding ...}" />
  </actinav:ZoomContentControl>
</local:Ruler>

...it breaks.  MeasureOverride() is called repeatedly, with an ever-increasing childElement.DesiredSize.Height.  Uncommenting the "WriteLine" above yields output like:

Desired Size: 584, 279
Desired Size: 584, 282
Desired Size: 584, 285
Desired Size: 584, 288
Desired Size: 584, 291
Desired Size: 584, 294
Desired Size: 584, 297
Desired Size: 584, 300
Desired Size: 584, 303
Desired Size: 584, 306

(....Indefinitely.)

Why would wrapping the content in a ZoomContentControl cause this type of infinite recursion/infinite looping-type behavior?