Fix drill picking ground primitives

CHANGES.md

@@ -17,6 +17,7 @@ Change Log
 * Added ability to support touch event in Imagery Layers Split Sandcastle example. [#5948](https://github.com/AnalyticalGraphicsInc/cesium/pull/5948)
 * Added a new `@experimental` tag to the documentation. A small subset of the Cesium API tagged as such are subject to breaking changes without deprecation. See the [Coding Guide](https://github.com/AnalyticalGraphicsInc/cesium/tree/master/Documentation/Contributors/CodingGuide#deprecation-and-breaking-changes) for further explanation. [#6010](https://github.com/AnalyticalGraphicsInc/cesium/pull/6010)
 * Moved terrain and imagery credits to a lightbox that pops up when you click a link in the onscreen credits [#3013](https://github.com/AnalyticalGraphicsInc/cesium/issues/3013)
+* Fixed a bug where drill picking a polygon clamped to ground would cause the browser to hang. [#5971](https://github.com/AnalyticalGraphicsInc/cesium/issues/5971)
 
 ### 1.39 - 2017-11-01
 

Source/Scene/ClassificationPrimitive.js

@@ -548,6 +548,7 @@ define([
 
         if (classificationPrimitive._primitive.allowPicking) {
             var vsPick = ShaderSource.createPickVertexShaderSource(vs);
+            vsPick = Primitive._appendShowToShader(primitive, vsPick);
             vsPick = Primitive._updatePickColorAttribute(vsPick);
 
             var pickVS = new ShaderSource({

Specs/Scene/ClassificationPrimitiveSpec.js

@@ -644,6 +644,25 @@ defineSuite([
         });
     });
 
+    it('drill picking', function() {
+        if (!ClassificationPrimitive.isSupported(scene)) {
+            return;
+        }
+
+        primitive = new ClassificationPrimitive({
+            geometryInstances : boxInstance,
+            asynchronous : false
+        });
+
+        verifyClassificationPrimitiveRender(primitive, boxColor);
+
+        expect(scene).toDrillPickAndCall(function(pickedObjects) {
+            expect(pickedObjects.length).toEqual(2);
+            expect(pickedObjects[0].primitive).toEqual(primitive);
+            expect(pickedObjects[1].primitive).toEqual(depthPrimitive._primitive);
+        });
+    });
+
     it('does not pick when allowPicking is false', function() {
         if (!ClassificationPrimitive.isSupported(scene)) {
             return;