Fix #6858

Author: hpinkos

Source/Scene/QuadtreePrimitive.js

@@ -770,8 +770,11 @@ define([
     var scratchRay = new Ray();
     var scratchCartographic = new Cartographic();
     var scratchPosition = new Cartesian3();
+    var scratchArray = [];
 
     function updateHeights(primitive, frameState) {
+        var tryNextFrame = scratchArray;
+        tryNextFrame.length = 0;
         var tilesToUpdateHeights = primitive._tileToUpdateHeights;
         var terrainProvider = primitive._tileProvider.terrainProvider;
 
@@ -782,14 +785,20 @@ define([
         var mode = frameState.mode;
         var projection = frameState.mapProjection;
         var ellipsoid = projection.ellipsoid;
+        var i;
 
         while (tilesToUpdateHeights.length > 0) {
             var tile = tilesToUpdateHeights[0];
+            if (tile.state !== QuadtreeTileLoadState.DONE) {
+                tryNextFrame.push(tile);
+                tilesToUpdateHeights.shift();
+                primitive._lastTileIndex = 0;
+                continue;
+            }
             var customData = tile.customData;
             var customDataLength = customData.length;
 
             var timeSliceMax = false;
-            var i;
             for (i = primitive._lastTileIndex; i < customDataLength; ++i) {
                 var data = customData[i];
 
@@ -869,6 +878,9 @@ define([
                 tilesToUpdateHeights.shift();
             }
         }
+        for (i = 0; i < tryNextFrame.length; i++) {
+            tilesToUpdateHeights.push(tryNextFrame[i]);
+        }
     }
 
     function createRenderCommandsForSelectedTiles(primitive, frameState) {