.net - Long Lines Don't Render Correctly

I have a project with a canvas that the user can zoom and pan using the mouse. The origin is marked with a blue cross running through the canvas. As the user zooms in the line width of the markers is reduced to keep the them about 1 pixel wide on screen. However, after zooming in about 10x the origin lines abruptly disappear. It seems like the rendering engine is deciding that the line is too small to be displayed before applying the scaling transform.

What is strange is that when I do the same operation to a rectangle instead of a line I get the expected behavior (see the ScaleBox rectangle object in the code below). Any ideas on what is causing the problem and how to fix it would be welcome.

Here is the relevant XAML:

<Canvas x:Name="MouseCapture" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Background="WhiteSmoke" >
<Canvas x:Name="ZoomPan">
<Canvas.RenderTransform >

[code]....

Here is a screenshot as 10x Zoom. The origin lines have disappeared abruptly, but the black box still shows up as expected. (Note that only one side of the black box is visible because the rest is outside the visible region because of the zoom.)