Polygon transparency changes as you zoom? (how to turn off)

I am trying to render some simple polygons on the surface of the globe in the 3 different perspectives (2D, 2.5D and 3D sphere). However, when rendering in the 2.5D perspective, the polygons do not at first appear and only "start" to appear as I zoom into the globe (see e.g., http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Circles%20and%20Ellipses.html with the 2.5D view and zoom in and out on the circles)

Is there some way to change it so that the polygons are visible at all zoom levels?

This is a known issue, see #92. We could help you hack out the lighting in the meantime, but it will only be a temporary fix until we are able to do the real one.