Add test

CHANGES.md

@@ -7,6 +7,7 @@ Change Log
 * Added `cartographicLimitRectangle` to `Globe`. Use this to limit terrain and imagery to a specific `Rectangle` area. [#6987](https://github.com/AnalyticalGraphicsInc/cesium/pull/6987)
 
 ##### Fixes :wrench:
+* Fixed an issue in the 3D Tiles traversal where external tilesets would not always traverse to their root tile. [#7035](https://github.com/AnalyticalGraphicsInc/cesium/pull/7035)
 * Fixed an issue in the 3D Tiles traversal where empty tiles would be selected instead of their nearest loaded ancestors. [#7011](https://github.com/AnalyticalGraphicsInc/cesium/pull/7011)
 
 ### 1.49 - 2018-09-04

Source/Scene/Cesium3DTilesetTraversal.js

@@ -443,13 +443,10 @@ define([
             return false;
         }
         if (tile.hasTilesetContent) {
-            // Traverse external tileset to visit its root tile.
+            // Traverse external tileset to visit its root tile
             // Don't traverse if the subtree is expired because it will be destroyed
             return !tile.contentExpired;
         }
-        if (tile.hasTilesetContent) {
-            return true;
-        }
         return tile._screenSpaceError > tileset._maximumScreenSpaceError;
     }
 

Specs/Data/Cesium3DTiles/Tilesets/TilesetOfTilesets/tileset.json

@@ -33,7 +33,7 @@
         88
       ]
     },
-    "geometricError": 240,
+    "geometricError": 70,
     "refine": "ADD",
     "content": {
       "uri": "tileset2.json"

Specs/Data/Cesium3DTiles/Tilesets/TilesetOfTilesets/tileset2.json

@@ -31,7 +31,7 @@
             20
           ]
         },
-        "geometricError": 70,
+        "geometricError": 0,
         "content": {
           "uri": "tileset3/tileset3.json"
         }

Specs/Scene/Cesium3DTilesetSpec.js

@@ -1079,8 +1079,7 @@ defineSuite([
         //
         return Cesium3DTilesTester.loadTileset(scene, tilesetReplacement1Url).then(function(tileset) {
             tileset.root.geometricError = 90;
-            viewRootOnly();
-            scene.camera.zoomIn(20);
+            setZoom(80);
             scene.renderForSpecs();
 
             var statistics = tileset._statistics;
@@ -1364,6 +1363,15 @@ defineSuite([
         });
     });
 
+    it('always visits external tileset root', function() {
+        viewRootOnly();
+        return Cesium3DTilesTester.loadTileset(scene, tilesetOfTilesetsUrl).then(function(tileset) {
+            var statistics = tileset._statistics;
+            expect(statistics.visited).toEqual(2); // Visits external tileset tile, and external tileset root
+            expect(statistics.numberOfCommands).toEqual(1); // Renders external tileset root
+        });
+    });
+
     it('set tile color', function() {
         return Cesium3DTilesTester.loadTileset(scene, noBatchIdsUrl).then(function(tileset) {
             // Get initial color