Cesium JS - geometry age determines visibility behind low-opacity billboards

gistfile1.txt

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;
var primitives = scene.primitives;

var old_col = new Cesium.PolylineCollection();
scene.primitives.add(old_col);

var billboards = primitives.add(new Cesium.BillboardCollection());
billboards.add({
    image : '../images/Cesium_Logo_overlay.png',
    position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883, 1110000)
});

var new_col = new Cesium.PolylineCollection();
scene.primitives.add(new_col);

Sandcastle.addToolbarMenu([
{
    text : 'Choose an item below',
    onselect : function() {
        
    }
},

{
    text : 'Add polyline to old collection',
    onselect : function() {
        var start_pos = Cesium.Cartesian3.fromDegrees(-43.679, 70.269, 1000000);
        var end_pos = Cesium.Cartesian3.fromDegrees(-81.62, 28.0, 1000000);
        
        if (window.new_polyline) {
            console.log('removing new polyline');
            scene.primitives.remove(window.new_polyline);
        }
        
        
        window.old_polyline = old_col.add({
            positions : [start_pos, end_pos],
            material : Cesium.Material.fromType('Color', {
                color : new Cesium.Color(193, 193, 193, 1.0)
            }),
            width : 1.0
        });
    }
}, {
    text : 'Add polyline to new collection',
    onselect : function() {
        
        if (window.old_polyline) {
            console.log('removing old polyline');
            scene.primitives.remove(window.old_polyline);
        }
        
        var start_pos = Cesium.Cartesian3.fromDegrees(-40.179, 70.269, 1000000);
        var end_pos = Cesium.Cartesian3.fromDegrees(-77.12, 28.0, 1000000);
        
        window.new_polyline = new_col.add({
            positions : [start_pos, end_pos],
            material : Cesium.Material.fromType('Color', {
                color : new Cesium.Color(193, 193, 193, 1.0)
            }),
            width : 1.0
        });
    }
}]);